because you don't have other options.
lots of people, men and women both, are only in a relationship because they don't feel they have any better options. they believe, and they might be right, that they will never find a future partner, and/or any future partner won't be as good.
a lot of people stay in crappy unhappy relationships because it's better than being alone or fear of being single.
and a lot of people have no model of a happy relationship to compare against. when i was young, in my hometown, literally nobody had happy parents. when i went to college i was SHOCKED at being around people who had happy parents and visiting their homes sometimes and seeing families that actually loved one another instead of HATED each other.