It's almost as if they partly lack theory of mind, and cannot imagine that somebody else does not know what they know, and consequently give explanations that are only suitable for someone who already knows the material.
Mathematicians do not find difficult to verbalize motivation and intuition because they (we) are autists. We struggle to do it because it is really difficult sometimes. You can find that you have an intuition that you cannot put into useful words (sometimes the intuition is "it should be like this"), or that it relies on more advanced concepts (something that clicks only when you get to know concepts that rely on it), or that you know that the idea in your head is not useful to others and putting that down in a book would make things worse. I have not written any book, only some notes for other students, and with that I already know it is difficult to write a good motivation and intuition to abstract concepts even when you completely dominate them. Moreover, most of the mathematicians learn to develop their own intuition and motivations and it is not as important to write yours (which are subjective most of the time) down.
I always imagine that skilled mathematicians are somewhat the same. It isn't that it only works for someone that already knows, it is more that they are writing the books for folks whose mind simply has that sort of slant to it.
If we change all the books to help someone like me - I generally did poorly in algebra, trig, and calculus but found geometry fairly straighforward - that's going to let the other folks down because their mind's logic simply doesn't work on the same wavelength. This isn't autism, but simply human variation.
I am not denying that talent exist, but it just makes learning harder or easier and defines plateau. Through, if you was good in geometry, then your brain works well enough in math (meaning some click on algebra did not happened but you are unlikely to be unable to think that way - thinking is not that dissimilar).
Moreover, mathematiciens are trained to remove things like motivation and intuition from scientific papers and when they communicate among themselves. You get punished when you dont. The training (which is perfectly sensible for given field) is much simpler explanation then attempts diagnoze people.
Why is this the case?
Teachers know that their students don't know everything. But the hard part is guessing what they actually do know and adapt to it.