Largely this is a class thing - writers tend to be cosseted little middle-class kiddies who think that the world owes them a royalty cheque. But just doing it - being in your room for years on end locked in your head alone with invented ghosts - it weakens and softens the body. And I know I can't just live in my head.
One of the most amazing things about mathematics is the people who do math aren't usually interested in application because mathematics itself is truly a beautiful art form. It's structures and patterns and that's what we love and that's what we get off on.