From personal experience, I was in long term relationships with two very nasty abusive people (2 years and 3 years) and the reason you stay is because they ware you down to basically just a shell, you have no fight in you anymore, nothing that makes you YOU. They control you and make you believe this is all you're worth and that you are nothing without them, whereas in reality it is the total opposite.
The cycle of abuse is a very powerful one, it can be easy to look in from the outside and say you wouldn't stay but you don't always realise it's happening until you're in very deep and then it can be hard to get out of.