Wellll, first of all those girls let them. What else can a girl expect out of a guy if she's willing to do sexual things on first "date". That makes her no better than they are. Also, guys that are only interested in sex could care less about a girl's "real" feelings, and the less emotion the girl show has the better. So to the guy, they don't think they're doing anything more wrong than the girl is. If a girl doesn't want to only be a hook-up girl then she should get to really know a guy first plus take the necessary time to develop some kind of a good relationship with him before considering a hook up. When they don't do that, then they have no one to blame but themselves, and will continue to be "used" and that means being hurt a lot and her ego shattered..
Many girls respond especially positively to jealousy (such as by competing with the other girls and chasing after the guy). If he constantly hooks up with lots of girls, there is probably little novelty to him in it all (it's normal for him). The fact that he feels less and is largely unaffected by girls' physical attempts to seduce him, along with his demonstrated value (other girls wanting him), will probably make them try harder to seduce him. That's attraction.
those guys are only looking to just have sex and they don't have any feelings for the girl that they are hooking up with and don't ever want to have a gf. That's the majority of the guys in college I have found.