The simple answer is - because I know there's no way in hell I would ever be the best. Not even the best from my home town. Why?
This guy - Terry Tao, just won a Fields Medal - Australia's first. To put this in context, I was always the ludicrously geeky kid who'd been skipped ahead in school. Terry made me look slow back then, and I met him enough times to know I just wasn't anywhere near his level. I guess no-one else in the world was, either - he got his Fields aged 31, compared to the other recipients this year who are 36, 38 and (Perelman, otherwise known as Professor Take-the-Poincare-and-run) 39.
Congratulations, Terry. I am permanently in awe.