Partially because you know it is not fully true. You have exaggerated the negative and minimized the positive. Maybe you realize that your life is not about you, it is about way more than that. If you stay married, you can benefit more people in more ways than if you were single/divorced. It is not just about passion and intimacy, good conversation and fun times. Maybe you stay because you believe you are trapped by life circumstances or you feel desperately lonely if not in a relationship.
So there is the struggle. We should have good times and fun together and yet it is so much more than that. We should have some romance or we are simply friends and should redefine our relationship. We should sacrifice. We should not put up with abuse, though. We should hold ourselves and our partner accountable to grow and develop, to find our purpose on this Earth. We should forgive one another and ourselves and attempt to not make the same mistakes over and over. We should love ourselves and our partner fully and wholly.
Relationships are really tough and a lot of work, and not just the romantic relationships.