Girls has your boyfriend ever hit you?Watch this thread
My mum was physically abused by my dad for years and it is easy to say "well why don't they just leave?" when you are looking in from the outside but its not always that easy. He threatened to kill her or himself if she left and for a while, the fear of him doing something like that kept her with him in fear and eventually she got away and never looked back. Even she says that its easy to look back and say that she should have got out a lot sooner but it is hard when you are scared and also when an abusive partner manipulates you.
It's very difficult to walk away because the abuser makes you believe that they'll find you no matter what, and they make you feel trapped and terrified of leaving. They can make you feel like the most special person in the world, and so when they hit you you desperately want their affection again so you feel special. They also can threaten to kill or hurt themselves, so you stay because you're worried you'll be to blame if they're hurt or killed.
Again, most people don't understand unless they've been in the same situation. (Source: personal experience)
For many, it's not a simple case of just packing your bags and walking away though. Some genuinely believe that their partner loves them because one minute they'll attack and the next minute they are apologising amd saying that they love them. It's sad.