Why do guys with girlfriends talk to me constantly?

It seems like so many of the guys that I've been meeting/talking to lately already have girlfriends, yet - they still talk to me. This one guy I met at a friend's party like a month ago and gave my number to texted me all the time saying "let's go out for a drink" etc etc. - never, not once, mentioned that he had a girlfriend. Then last week Thursday I get a call from his girlfriend telling me to stop talking to him... Uhhh, he never even mentioned her! Then there are a couple other guys that text me randomly that have girlfriends. I understand that we are like friends or whatever, but they kind of flirt with me too.... I don't get it.