Because most girls fall into 3 categories:
1. Ugly or Fat. Guys do not want these girls by choice because of what their friends would say etc
2. Generic, usual girls. Guys want that 10/10 of their dreams and tend to ignore this group or stay with one temporarily (for sex/ experience/ companionship) whilst looking for the perfect 10 aka a whole bunch of cheating happens here when they find someone more suitable to their 10/10 ideal woman look.
3. Beautiful. Guys are intimidated thinking she will reject them or has a boyfriend already and thus don't try.