Exactly. Some girls have low self esteem and think thats all they will ever get, someone who treats them lke shit and they settle.
Speaking from experience, its not because we females are crazy or love drama. Most men start out like prince charming and eventually turn into complete assholes, but we love them and still remember when it was a perfect relationship. That's why we defend them and stay with them because we know it hasn't all been bad, and most of the time when they are nice and things are good, they are really good. That makes us not even want to think about the bad. So don't become an asshole, at some point she will hopefully come to her senses.
Some guys start out nice and then get mean and the girl doesn't have the courage to get out of the situation... or they keeping idealizing and sticking to all the good qualities of the relationship, despite the fact that it's actually unhealthy. or, some women just have deep seated issues and tend to pick awful boyfriends, because they think they 'deserve it', for example.