Mr. Answer Man says: Because they work. The thing to remember is that the guy doesn't care what you think about the line -- he wants to know what you think about him.
If Mr. Answer Man walks into a bar here in New York City, approaches every woman he makes eye contact with and says, "Hi, I'm Mr. Answer Man. What's your sign?" what he'll get mostly is the brush off. But eventually, some nice young lady will say, "Oh, that's so dumb," but she'll smile when she says it, and when I ask her what her name is, she'll tell me. And that's what I'm looking for -- a woman who's predisposed to like me. If I'd wasted time proving myself in scintillating conversation with all the others, I might never have gotten around to finding the one who thinks I'm cute just because.