Cause it's the anime.
Seriously, no reason; it's how the creators wanted it. That's how they wanted it. It is to balance the anime. Otherwise some might be overpowered( apart from the obvious, Tobias, i.e. the Darkrai guy), and others might just sit doing nothing at all, just coming to lose. No fun in obviousness, is there?
Also, people like youngsters to fight, show enthusiasm, and ultimately, win( though Ash has not won much), not 20-year-old grown up men fighting with others like a maniac.