Two different sets of circumstances.
The one who is being abused was abused as a child and unfortunately equates that with love, and their response to abuse is to try harder. If only they would love enough, then they would be good enough, then the beatings would stop.
This is child logic. It is a coping mechanism developed as a child, and has to be unlearned. And it is a painstakingly slow process
And yes, these people get more attached the more they try.
(btw, a normal person will just leave as soon as abuse starts.)
Relationship sunk cost fallacy is almost the entirely the pervue of men. Men have to screw up their courage to approach a women, and then they invest the time and emotions into that woman. If they were to leave, they would be giving all of that up.
Women, especially attractive women, get approached all of the time. Many women have an alternative man waiting in the wings.
Women's hypergamous nature also insists that she trade up if possible.
So, to a woman, they do not consider the relationship equity as anything important.