1. He dislikes what he perceives to be the corrupting effects of money and wants to guard against their influence in his life.
2. He thinks about pure math all day, and this makes him happy. Has solved for his basic needs. Does not want to worry about managing $1M. If and when he needs more money, he will deal w/ it then. Since he is a famous mathematician, it is reasonable to expect that others will help him out, if he asks.
These are variations on theme, actually. He enjoys thinking about math. He doesn't need $1M to think about math. Ergo, he doesn't need $1M.