How can a woman

I’m guessing of course but she may not love herself and doesn’t feel worthy of someone who treats her well She may feel on some level that she deserved to be treated that way and will keep trying to gain his approval

She may be financially dependant on him. If they have children together she may not want to take them away from their home or not have anywhere safe for them to go.
She may be scared - often the risks of abuse or worse rise massively when a woman tries to leave a controlling or abusive relationship.

I can see that. Even if he doesn't abuse her, but is just a complete asshole and jerk toward her, she would have to have a low opinion of herself.

There can be so many reasons for this. @FlyingHome gave some excellent responses. Another reason is this: as adults we often seek out the love we saw as children. If you grew up with a father who was abusive towards our mother and the mother stayed, that's what love looks like. With our adult reasoning we know it's fucked up but on an emotional level we might seek out that sort of relationship. In fact we often do. That's what I did.