Maybe because they already have girlfriends. I'd say ignore them back and don't acknowledge them.
Well its 2 things.
1. Guys don't require constant communication. Many times a girl thinks a guy is ignoring her when it his mind he's not, because going a day without talking is a big deal to a girl, to a guy its not.
2. Guys who are good with girls, employ this technique. It's hard to do since we naturally DON'T want to ignore the person we really like. Guys who good "game" know that this idea of planting the seed then ignoring makes a girl go crazy for you, like it is now. You don't realize it but this behavior makes you almost want him more. It has to do with the fact that by him ignoring you, if places his at a perceived higher status. You all of a sudden wonder why he isn't interested in you and subconsciously you reason that its because he must be of higher status that you. This in turn makes you want to win him over even more because women are innately born with the desire to mate with the highest status men, aka the alpha males.
Well, this usually happens to me because I try to flirt with a girl, and she doesn't reciprocate at all. And I feel like a miserable failure in ever trying to get her attention. And I stay away from her, because she isn't interested in me in the slightest. And that's why I flirt and then ignore.
because guys love to flirt.:)