Its simply because they're hot. They aren't potential mates. Chances are 95% of the women whom have the hots for them will never meet them and there is almost a guarantee that these women will never be with them. Therefore, these men are just nice to look at.
Omg I didn't know Zac was short up until now well I don't really care because he's still hot I want to say the same about Justin and especially in the pics you just posted but god he looks horrible right now and needs a hair cut -_-