One reason I haven't heard yet is that the work gets harder.
Before I went to Oxford, I'd never run into anything mathy that I couldn't understand. I don't mean that I got it right the first time, though that tended to happen, but in high school I either just got it with no problem, or I understood the answer when I saw the explanation. This goes for both math/science class itself and math contests that I enjoyed doing. No real mysteries, either trivial mistakes or some angle you hadn't seen, but well within reach.
Then in uni I would run into problem sheets where I not only could not get the answer, I couldn't even find a page in the ten books that seemed relevant. Neither could my study partners. I'd show up at the tutorial and the tutor would throw up some equations that made it seem like a five minute job, when I'd sat in the library for 5 hours having discovered nothing. I'm talking something like a 10 problem sheet where between me and my buddies we managed 2 correct answers, 2 somewhat close, and 6 complete mysteries.
This can be a shock. People react in different ways, and one reaction is to not try.
In the end the thing to do is... study harder. When you understand these problems in depth you are no longer doing an equation fishing expedition, you actually understand what you are looking for.