When a girl is abused you have to realize for something like that to get to that point, this guy has done some serious psychological work. So he is telling her how much he loves her, and will take care of her, and how much she needs him. He will probably reinforce that a lot, that she needs him. So she will start to believe these things. When he abuses her she becomes confused but she's not going to leave because now she's scared. If someone can love her so much why is he doing this? So he will apologize and she will believe him because he tells her how much he cares about her. And besides if she leaves it's just going to make him mad and do worse work on her. If and when she finally gets out, if she can finally get away she doesn't know what a normal relationship is, and all she knew was there was a guy who hit her but he loved her, and although it doesn't make sense and shouldn't she doesn't know much difference, so she is attracted to these men that do that because that's the only way she feels cared about.
Women become dependent on their abusers. You get used to a certain way of life and become accustomed to living that way. If you live with them sometimes you rely on them for food and shelter or to provide for your children. Maybe they have been abused by someone growing up, or maybe they are insecure and don't feel they deserve more. There is also a type of person in a relationship called a caregiver (an enabler) who is so consumed with taking care of another person that they will deal with a lot in order to just keep the relationship going. Some people feel like they failed if they can not make something work so strieve harder to please this other unpleaseable person. A caregiver is a nurturer like a mother and will stop at nothing to keep their relationship. It is very self destructive behavior, but there are many people out there that do this. I know from my own experience. It's easy to get used to something and attached. Many people will point fingers, but no one knows how it is unless they go through it personally. I feel for you if you know anyone going through this way of life, do your best to be a supportive friend or family member if that is the case. There is a lot of information on the internet about this sort of thing. If the relationship is physical make sure you report anything to the police. Your friend/family member will thank you later, it just might save their life.
Because their self esteem gets totally shot. So they think that it is their fault, not his. They want to do something right so he won't hit them. And lots of times the abuser will have told her she is lucky he stays with her or nobody else would like her etc. Usually when there is physical abuse emotional/verbal abuse is present as well.