Young girls are much easier to manipulate and control, and they are less likely to push back and voice their opinions. Sure, people are different, and there are strong and opinionated 18-year-olds. But it's rare.
Also, a 30-year-old man is a "regular" guy for a woman of the same age. But for a girl of 18 years he's a HERO! He has a stable job, he's doing well, he has life experience, he has goals and he's much smarter than 18-year-old boys surrounding her.
It's as simple as that. Men want to be "heroes" and extra special. Plus, dating a much younger girl = easy manipulation and control.