One word: convenience.
They like being in a relationship, and enjoying the perks that come with that-sex, companionship, status...etc.
I think it's fairly obvious what the man is getting out of this situation. What is unclear is what the woman is getting from it.
I think the more pertinent question is why do women stay with men who aren't giving them what they want? I get that you hear what you want to hear so when he says, "Not just yet," you're hearing, "Soon," when it should be, "Never."
I think more women would be better off if they set themselves deadlines in these kinds of situations. I'm not saying give an ultimatum, but be clear about what you want, and be prepared to leave if they're not giving it to you. None of this-oh, I'll just hang on for another year when he gets this promotion, we make this and that move...
Women don't get the luxury of hanging around for years after a man if she wants children. Biology isn't on our side. So, if kids are on the cards-it's time to think forward and make moves in line with that and not waste precious time on men who will never give you what you want.