I notice this just about every day. Every time I go to a grocery store, a cafe, anywhere really, anytime I see a pretty girl, 9 times out of 10, the guy she is with is either average looking, or below average looking. He may be noticeably short, or noticeably fat, or both short and fat. Greasy hair, poor or nonexistent fashion sense, or just flat out looks like a bum.
Even though I work out, have a nice body, nice clothes, nice car, am polite, good job, I have a hard time getting the prettiest girls to date me. This used to frustrate me and still does. I think what is happening is that girls want to be in a relationship, and the best way of insuring that she stay in one, is to find a guy who she thinks is GRATEFUL to be with her. That means, a guy who is not so good looking, and therefore (she believes) less likely to cheat on her.
In the meantime, guys who she thinks have A LOT of dating options (because of good looks, for example) are almost frowned upon by beautiful girls because they fear these guys will bring the drama, cheat, or dump her for a better looking girl.
Also, there have been times where I have been really nervous because I thought a girl was very beautiful, so I know what these girls are feeling.
Anyway, while it is frustrating to see beautiful girls choosing ugly guys, broke guys and abusive guys as boyfriends, at least I have a better sense of why.