Why can't women dump bad boyfriends?

Why don't you leave your boyfriend?

I was reading some boyfriend ratings recently, and I found myself shocked by the similarity of many posts. Girlfriend after girlfriend being treated poorly by inconsiderate and, often down right mean, boyfriends. Yet, their girlfriends keep putting up with it. Why?

Are the boyfriends in question not really that bad? Are these just crazy girlfriends that love to complain? Maybe, but I'd bet many of these women are just putting up with bad boyfriends. Is this because the nurturing nature of women pushes them towards bad men? I don't get it.

Why do women stay with bad boyfriends?