It's pretty simple - marriage, even an unhappy one, used to be the best life choice for women. Only recently has this changed. For most of modern (and premodern society) marriage was the only source of security for a woman and her children, it had to be pretty terrible for her to take the alternative. Which was often unthinkable for the 99% who weren't rich.
(By contrast, an "unhappy" marriage remains the best choice for most men, who get significantly worse outcomes in terms of mental health, physical health and life prospects after divorce, than if they remains in an unfulfilling marriage.)
Now the alternative is fine. Not ideal, but fine. You can survive and be happy on your own. So why stay in an unhappy marriage?
Most marriages go through unhappy periods. Exciting strong emotions always fade. New things always become Boring. We all get older, less attractive, less energy, less excited. So when that happens, why not leave?
When the women isn't hot enough, why not leave? When the man isn't caring enough or useful enough? When he doesn't do his share, or she becomes unpleasant and selfish?
there is still a few things keeping people together.
Some people genuinely get lucky and end up with their "soul mate". Some people are very logical and pragmatic and work through every issue. Some people are religious or still have a strong stigma about divorce from their families or social setting. Some people do it for the kids some people, for their own reasons don't have a good alternative and are trapped.
But the truth is that for a lot, staying in a situation where you are unhappy simply doesn't make sense, if the alternative is better.
My contention with modern society though is -
We can't go back to a situation where everyone is married forever. But we know that in current society kids without married parents fare worse of (not their parents fault). So how can we re-think childcare as a society so that it isn't all 100% focused around a method of life (marriage) that doesn't work for a many any more. we can't keep organizing childraising around the idea of two present parents if there isn't always going to be. 2 present parents hasn't always been the norm across cultures and history. There are other ways we could consider arranging our society and communities.