It is very difficult to explain something which was obvious to you. Often you would not know how to decompose it, and above all, you would not know the pain points the others encounter, and why those points are painful. Teaching is a trade, after all.
This is explicitly why I did so well at Maths at A-Level. Once I could explain something to my less gifted peers, I had it. Completely and without the need to do the endless repetition I found so boring that is otherwise normally used to get good at solving those types of problems.
