Because they're young and have a lot of growing up to do.
Because they're easy to get. They are young and are not trying to settle down. They're just trying to have their fun.
Because there easy to get to. They look for the attention.
Because they're attractive. All guys like attractive people. And because guys are often looking for sex in a relationship and want to get with a girl that can give that to them.
Because they are willing to do anything and easy to get with.