Why do people stay?

I’ve been going to therapy and one of my “homework” assignments are thinking about why people stay. Whether it’s in a friendship, marriage, job, etc. For example, my mother has been in a physically and emotionally abusive relationship for years (my father) and she has decided to stay in the marriage for reasons I have yet to understand. She has the resources to leave, she can buy her own house, so it’s definitely not a money issue. It would be great to hear some different perspectives on this topic.