Abusive Relationships.

I've seen so many posts and situations back to back where there's a person who is in an abusive relationship, and knows that they are in one - but decide to stay anyways. ...... For example, the couple have been together for a few years and one party is abusive to the other. The second party knows that their partner is abusive and knows that it's wrong... If they know it won't get better, and that it's wrong. Why don't they just leave?