What makes people stay in a relationship when they are miserable and there is no children involved?

my sister has been with this guy for 3 yrs and all she does is complain about him and that one day she will leve him but never does.i will say he is a loser he has a clingy exwife,bratty spolied kid($$)=0 money so they never do anything,he spends his free time playing video games when his kids not over.Has cheated on weman in the past.Plus he treats her like a doormat/maid and all ways tells her hes leaving her when things don't go his way but never does.Now my sis has 3x his money(why i think he stays with her)"no baggage",not ugly or fat so why does she stay when she is so unhappy in this relationship?