Yea, many 'popular guys' may do this to show that they can have any girl, it may just be a 'guy thing' amongst themselves that make them feel good or wanted?!
Guys feel like they need plenty of girls so that way if they lose one, they'll have another waiting for them. I believe that guys are like girls in that they don't want to be alone. And these guys that are popular are afraid of not having girls with them because they think it'll make it look like girls don't like him and he's unattractive. But by having lots of girls around him, it'll make him feel better and like he does look good and boost his self-confidence. I think it's just a problem that a lot of guys might have. Once a guy finds that one girl he truly loves, he'll stop worrying about the other ones. Trust me because my boyfriend of over a year gives his full attention to me rather than worrying about him appearance or other girls. Hope this helps! :)
That's a big generalization... Not all guys are the same, so if you want a relationship, look for a guy who wants the same thing.