Sidney, I would guess you are asking why people don't call when they say they will. Or agree to something that they have no intention of doing. Etc. Etc.
I think it is because they don't want to appear disagreeable to their new friend.
But, for what it's worth, it isn't only in the dating scene. Friends, spouses, lovers, kids, coworkers, will all say whatever you want to hear, when it is obvious they don't mean it.
Now why? I have no real idea, and like you, I find it really irritating. I have learned to call people on their answers too. So when someone says to me "Let's get together next Saturday; I'll call some people and we'll go out to eat.", I will, depending on the person, say "We aren't going to do that" or "That's not going to happen."