I think there's so many aspects that add to it & its different for each person too.
Men can be like this too (I know alot of men unhappy in their relationships - but they stay) - I think the difference there though, is men tend not to talk about their issues/what's bugging them. Whereas women like to talk and discuss things.
My female friends will literally call me/message me whenever something happens between them/their partner and they like to 'offload' what's bugging them, every little detail and whatnot. Whereas my male friends won't, unless I see them in person and say 'how are things with ' and it's usually just a quick 'ugh the same as usual' and the conversation swiftly changes.
I think as humans, from a very young age, we naturally crave acceptance and approval from the people around us.
However this ranges from person to person, each person has their own tolerance levels to different types of behaviour.
And I think a lot of it boils down to the fact that more often than not - the good in a relationship actually outweighs the bad, but we don't often hear of the good. People are much more likely to speak of a bad experience they've had than a good experience (this is applicable in so many situations too, like with customer service, people are more likely to approach a manager to complain, than they are to approach a manager to compliment).
People also get used to things, they get comfortable and get used to routine. The thought of a big change scares them.
But everybody is different, some people have an absolute 0 tolerance for bullshit, whereas other people are too lenient with another persons behaviour. I think upbringing can affect it massively too. If you get somebody who's been emotionally abused growing up, people have a flight or fight reaction & this can be carried into their adult life.
It's a very messy subject with so many grey areas, there's no easy answer to be honest.