Who said we were? Other than a couple hardcore martyrs here, most of us stay where we are because we are happy in our homes. Being capable of tolerating a partner's quirks or bugs is a virtue. We don't flee at the first sign of trouble, or our partner supported us when we stumbled and we are grateful, or we are determined to improve our lives and sometimes that requires some new ideas or some inspiration from others.
Look, a lot of us humans have it pounded into our brain that sex is for sinners, or for kids, or a weakness. It takes some maturity and perspective to reach the conclusion that more love is better than less, and some steely resolve to own our sexuality in the face of a chilly spouse. This is no different than having political or religious differences, not much different from the work it takes to find common ground when you're upbringings were radically different.
Here, I'll explain my station. Husband saw me through cancer twice though no one in his family is ever sick and they all live to 100. He has watched me raise two little ones like some granola crunching hippy, breast hanging out and nude babies tearing through the spindle for 18 months, twice, though his own mother is a conservative, angry, uptight proponent of formula who was horrified by my tits. He has loved me when I was painfully skinny, bald, puking, and even when I spent more than a year furious at him for failing me in the bedroom. Before me, his life was easy and full of endless pussy from wild bar girls, because he's good looking, strong as a bear, and charming as hell. He could have left ten different times when things got hard, but he didn't. He stayed because we've got these great kids who are affectionate as Care Bears, because I have a cool little farm near the mountains, and because I'm a natural mom crazy about my family, when I'm not sick, I'm kind of hot. Okay, I'm kind of ordinary, but I'm in great shape and I smile all the time and I swallow. We have this great thing to share: this family and these happy kids. And we like one another and are mostly accepting of one another's flaws. And we're working on it.
As for my past, I spent the first two thirds of my life getting pounded into the mattress in a series of passionate long term relationships with beautiful, successful human beings I had no future with. I loved a hopelessly addicted stoner/musician who couldn't carry a conversation after 8 at night. With a quiet, unsmiling man who seemed the be angry under the surface all the time. With an obsessive compulsive monster who stalked my every breath because he was determined to love me to death. With a beautiful artist who tattooed my handwriting all over his body and pierced all his shit every time we argued. And finally with a lifer Jehovah's Witness who couldn't quite shake his raising and was convinced that everything except my pussy was demon-haunted, when it's far more likely that the reverse is true.
TL;DR version: because love. Because most of us love each other, and those of us who do describe misery or self-neglect, well, they do leave and they do so with our whole-hearted encouragement and well-wishes. Those of us who stay have every intention of making the most of things and helping our partner reframe sex so it's good for both of us.