I had an Asian calculus prof who was apparently brilliant, an important person in the field of superconductors iirc, but goddamn he was a shit teacher. He'd attack the board with his chalk, pieces would often fly everywhere. He'd attack the board with the problem, pause, attack the board with the solution, and tell us "Obvious!" like Mickey Rooney in Breakfast at Tiffany's might say it. That was how he taught everything.

I was told that in order to have his real work funded at the University he had to teach classes.

Thank God for that man's TAs where we had to go to learn the obvious.

I think this applies to what u/jet199 wrote, sometimes brilliant mathematicians have to teach, although I'm not sure based on my experience they necessarily should. And it was kind of a toss up who to reply to since it seemed relevant to both comments.