Because humans never experience anything in and of itself, but only the output of the interaction between sensory data and a brain, literally everything is purely the result of human mental activity.
That doesn't mean the external physical world doesn't exist, the information we use to construct that model must come from somewhere, and we can deduce that the source is a persistent and consistent one.
The philosopher Husserl said: “The tree plain and simple, the thing of nature, is as different as it can be from this perceived tree as such, which as perceptual meaning belongs to the perception, and that inseparably.”
He came up with the idea of the noema which is our experience of something, and noesis which is our conscious act of perception. For me, that's our act of interpretation of our sensory perceptions. Sometimes this all goes wrong and we construct a flawed model that does not correspond perfectly to actual external reality, such as when we are deceived by optical illusions, stage magic or just hallucinate. Fortunately we can test and correct our perceptions through action in the physical world.
I'm an out-and-out physicalist but I think he is quite correct, we must distinguish between our internal perception of things and how things actually are. Fortunately science is extremely powerful in this regard. It has allowed us to decouple our model of the world from the limitations of our perceptual system, and come up with rigorous models of reality such as Relativity and Quantum Mechanics that are not tied to direct interpretation by our perceptive systems.
When we look at a tree, it could very well be that the source of the tree is very much like the tree we experience, but it could also be wildly different. When we see a tree in a video game, we know there is no real source tree just like it, just ones and zeroes. I disagree that science fixes this problem. Tools are still just measuring the physical world. For example, if you used a tool to measure some aspect of the tree, you are still measuring the representation of the tree in this world. If I use the video game analogy again, my point is that you wouldn't be able to see true underlying 'source code' of the game tree by looking at it in the game.
I don’t expect any description to accord perfectly with the reality of the object it refers to. It’s just a description, which may be more or less accurate or useful. Science, and investigation in general, is a way to test and improve such descriptions.
This gives an argument about intuitionism: If you say that math is a byproduct of our wetware and nothing else, how come we can successfully teach it to turing machines, and have that process fill us on some holes we had in our understanding of maths, but not terribly large holes?
When you take a picture of an apple, you have a picture of an apple, not the apple itself. Both are real and related, but not the same thing.
Well, of course, they kind of can. There are drugs that make the world look like it's squashed, such as ketamine. The way that we perceive reality really is totally dependent on the properties of the observer. Of course we all, with our sober minds, assert that we are perceiving the ruler the "right way", but all this means it that we perceive the ruler in a way that most humans agree with. Jumping from that to "this is the way that the ruler looks for all possible subjects" is a leap of faith.