A few things probably play a part, like the fear that they're too old to start again, the fear of being judged, concern over finances and shared assets, concern over lifestyle changes, etc.

Also abusive partners aren't usually shitty all the time, a bit of good mixed in with a lot of bad can be a very powerful psychological combination (see the Stockholm syndrome).

Interestingly it turns out the people that make the most love-filled posts on Facebook and are always sharing their ideal happy life pictures are often some of the most miserable in their relationships. Like there's something about being in a shitty relationship that makes people want to advertise how happy they are, like a form of wishful thinking.