There are services like Turo, where you know the exact car you're getting, but, sadly, they're far from universally available. And it's pricier, of course.
All rental agencies surely know what fleet they have - typically it's as uniform as possible, to make the maintenance easier. Yet, it's not exactly unusual to see that a particular location has a fleet of entirely different make than they had listed on the website. Or, e.g. I know a place that doesn't have "compact" cars (or have so few of them I've never seen one on the lot), and just automatically "upgrades" everyone to "mid-size" - no clue why, they're a good (aka not scammy or too blatantly overpriced) company otherwise.
My only guess is that no one complains or cares about it.
Obviously, if all you care about is getting from point A to point B somehow then you're probably don't care much.
However, different cars have different characteristics, such as safety ratings (how likely you're to stay alive if you crash is important factor for some people), instrument clusters (my personal pet peeve - most cars have very poor UI/UX), rear window and mirror usefulness (in some you can get a clear view, in some you can barely see if someone's directly behind you or not), and all sorts of "luxuries" such as additional convex mirrors, backup camera or smartphone integration (CarPlay/Android Auto).
All those things - save for safety ratings - are quite subjective, so while some folks may just shrug and wonder how those matter, they make a lot of difference for others. The difference between a car that can drive and a car than you can drive pleasantly (or confidently) is probably significant.
But maybe I've also been spoiled by German car rental places — literally every single car I got here in the past ~5 years has had CarPlay, backup cameras, etc.
And I'd care about all of the above if I were to _buy_ a car, but for a week (or two) long trip, I genuinely can't imagine caring.
(Though, I guess, I am quite opinionated when it comes to picking my computers and phones, etc; and if I had to use a loaner Windows PC for a week I'd probably be mad too. I guess people can have that kind of preferences in cars?)j
I charged the company back via my credit card provider, and have never even considered doing business with that company again.