Maybe because gravity isn't a force at all but a consequence of the curvature of spacetime?
Moreover, quantum mechanics comprehensively lacks gravity. It isn't just "oh, well, maybe this system is also accelerating due to gravity but we can ignore it". It's, if atoms are in a superposition of "over there" and "over here", what is the gravity of the resulting system? We know we can produce such superpositions in the real world, and when those systems exist, there must be some answer to that question, even though we don't know what it is. What impact does very, very high gravity have on the non-superposition-related aspects of the standard model? How do energies of all the various bits of the proton change in such a field? And so on.
It doesn't help at all to simply reiterate the theory of relativity. The conflict between relativity and QM is deep, real, and very very challenging.
How can we ever really know that? Isn't the best we can do is just models?
I have daydreamed of becoming a physicist just to try testing this.
To directly answer your question, by experimental results.
Luckily, we’re still pretty far from a model which fully describes reality, and it may be entirely unobtainable for us. (Also, we only assume that spatial and temporal locality isn’t a thing, i.e. that the laws of physics are the same everywhere and everytime, so there’s that as well.)
Right now we can't say what gravity is. We know we don't have a complete description of it, because the contradictions between QM and relativity are something explainable to a motivated high school student, as they are pretty fundamental. You don't need to deeply understand either theory to understand enough of the conflict to know we don't have an answer.
This isn't a criticism of either theory per se or the scientists involved. They're the best we've got and they're each on their own pretty good. But we are as justified in saying "gravity is definitely a curvature of spacetime because Relativity says so" as we are justified in saying "gravity doesn't exist because it doesn't appear in the standard model at all". Both are basically the same statement relative to the theory they relate to, it is just more obvious the latter is wrong.
Who knows, maybe in a distant future we'll that that 4th fundamental force is not so fundamental after all.
Physics, as a natural science, deals in “if we do this then we observe that”. Note the “we”: it is inherently subjective. The models resulting from that are, similarly, a product of our minds seeking analogies and stories. Those models are useful for coming up with more experiments but are necessarily simplified maps, and if you have a map that exactly corresponds to the 100% of the territory then the map you have is A) useless, and B) impossible (so if you think you found one, you ought to think again).
At best you will never find such a foundational description of reality. At worst you will believe you’ve found one and be angered at anyone who chooses to use a different map and dares to not treat yours as gospel.
Just shut up and compute!
Nobody has yet created a fully convincing model, but it certainly would be elegant if spacetime itself arises from something that doesn’t need four-dimensional semi-hyperbolic space to be assumed.
Consider this: you have two massless particles that are motionless with respect to one another. Suddenly both particles acquire mass at the same time (let's set aside this is physically impossible - this is a thought experiment). How does curved spacetime explain the sudden gravitational attraction between those two particles and how they're accelerating toward one another? It doesn't.
These problems are how we know GR is wrong, the problem is it's not flat-out wrong! Einstein's field equations are better than what Newton had provided and explains more observed phenomena - they even predict phenomena we have since observed! GR is a very good theory (model) for gravity, but it's not the whole picture - and we know it. We simply don't know how to improve upon it and I personally think that believing curved spacetime is actual "reality" is a big part of why we haven't made much progress. Well, that and some other issues QFT has when attempting to model a graviton, but this comment is already long!
EDIT: merely(!) - this isn't to understate Einstein's contribution to the matter which was quite considerable! But when the old genius himself is warning you to not get too caught up in the model, I suggest we take heed.
EDIT 2: QFT has issues modeling a graviton, not a gluon.
You can't take an model, apply an impossible scenario and then claim the model is invalid because it can't account for this scenario that you admit is impossible.
That's not what's being said. The fact is GR cannot explain why these two particles will suddenly start moving toward one another. All I've done in this simple thought experiment is eliminate every other externality.
The key point here is GR doesn't explain why curved spacetime causes objects to move. It only says that the movement can be modeled by curved spacetime. This thought experiment was just a way of expressing that.
Which is all to say GR doesn't explain what gravity is, it provides us a (complicated!) set of equations for determining an object's motion in a gravitational field. In my mind that's a fundamental shortcoming of GR as it was a fundamental shortcoming of Newton. NEITHER of their theories even tried to explain what gravity was, they provided a mathematical model for describing the effects of gravity. Newton's was good, Einstein's is better - but neither tell us much about gravity itself.
OTOH, quantum gravity tries to tell us what gravity is, but that has run into issues. Not only is it going to take someone of the caliber of Newton and Einstein to figure this out and people like that only come around every few centuries, but we have to have the means to test the theory. We have several theories, but we don't have the ability to test which theory is correct. We simply don't know which way to go.
Anyway, all that is to say GR is incomplete - and we know it.
Imagine we explained it as "Mass causes X, and because X causes spacetime to be curved, we have that, mass (indirectly) causes spacetime to be curved." . But then, why would you be any more satisfied with this? Why would you not then ask, "Well, why does X cause spacetime to be curved?" ? (Or, "why does mass cause X?")
Now, perhaps there is such an X, and perhaps it will be found. But, I don't see why this would satisfy you any more than "Mass causes spacetime to curve" satisfies you.
If you keep asking "why", you either eventually end up in a loop, have an infinite regress, or stop getting an answer. A non-empty directed graph either has a cycle, an infinite outgoing path, or a vertex without an edge away from it. (And also, either a cycle, an infinite incoming path, or a vertex without an edge going to it.)
Many are content to allow the laws of physics to just be, without explanation, others may say that the laws of physics are explained by God, who is without explanation. Personally, I go with the latter, but, unless one wants to go with infinite regress or a cyclic explanation, one has to allow that something is without explanation.
For your complaint to be compelling, I think you should give criteria for, what conditions would something have to satisfy, in order for it to satisfy you?
(Also, you seem to assume that if the two particles suddenly acquired mass, that they would immediately begin to feel a force between them. While the hypothetical is presumably impossible, still, what I imagine happening would instead by that there would be a light-speed delay between when they gained mass and when they began to accelerate towards one-another. Though, I'm not sure if there is a fact of the matter as to "what would happen in this impossible hypothetical".)
For example, take the famous equation:
E = mc^2
E = m
Soo… by my interpretation:
Mass, energy and spacetime curvature are the same thing! They’re not “proportional” to each other and one doesn’t “generate” the other like an electric field by an electron. Instead, everything is literally made up entirely of space time curvature. That’s what matter and energy are.
This is why all forms of matter have masses — the only other option is empty space with a flat curvature, but that’s just the absence of matter. If everything else is curvature, then they must cause long-range distortions — which we call gravity.
Roughly, my concept is conserved quantities are topological defects, which is why they seem to have a neat “algebra” and integral quantities.[1] Conversely, mass comes in fixed but non-integer quantities because the total curvature of some complicated knot doesn’t have a simple algebraic expression.
[1] Makimg the fractions disappear is easy. Just multiply by the denominator. E.g.: we say the electron has a charge of -1 because we discovered it before the quarks historically. If the quarks were discovered first, we would assign it a charge of +3.
Natural units are very useful when doing math, but they don't reduce constants down to plain numbers, they still retain their units. Once you factor this in the rest of your argument falls apart.
As a hand-wavey example, one could claim that energy in the form of bosons is just a "wiggle"[1] of the spacetime fabric, and that matter in the form of fermions are topological defects or knots.
That way they're different enough that you'd want to use different units to represent them, but at their core, they're both distortions in spacetime that must inherently cause a distortion in spacetime at a distance (GR).
It also explains how they're inter-convertible. E.g.: energetic gamma rays can be converted into electron-positron pairs. If they're both "made of distortions" then it's like a very strong wave creating a pair of vortices spinning in opposite directions. You can't count the waves (it's smooth and continuous), but you can count the vortices.
[1] For the wiggle, imagine taking a huge sheet of cloth laid out flat over a smooth surface. If you tried to put a wave into the middle, the edges of the cloth would be pulled in. Contrast this with the typical view of fields as "vectors on top of a base (flat) spacetime", much like a mathematical function graph.