Because cool and attractive women mean that men have to grow up.
It's all about taking responsibility for oneself and establishing priorities. Most men don't like to do that (IMO), and often it takes the right woman to open men's eyes enough to enable that. Also, men have to want to have things like children, stable relationship etc, and that doesn't happen to most men till about mid-30. Well, it happens, but we don't generally take responsibility for it till later. Women are right into that shit from 25 on. If it doesn't happen, a dirt bag evolves.
Hard to explain on teh typewriter after 2 beers.