for me in this new chapter of my life, I ain't settling. I believe most stay for any one of several reasons. Here are some of them:
it's more difficult to leave than stay
fear of the unknown
don't want to be alone
at least they know what they've got w/ this partner
thinking they need to stay together for "the kids"
unwilling or fearful of taking risks by leaving
and some people are aren't even aware of the situation as it is
this isn't an all inclusive list, just some examples.