This is much more common than many folks believe. When we are young, we don't often see a person for what and who they are. We are a bit more visceral at that age. Sexual attraction, "chemistry", wanting a relationship, marriage, children; the whole conquest is so much more important than finding someone who meets the criteria of mutual respect, common interests, kindness, compatibility. We don't always recognize that the person we married has deep psychological issues, as we are inexperienced and not trained in psychiatric issues. Sometimes we even believe that it is our fault that things are not going well so we make the effort to "work things out" because by that point, we already have children and as such, we have been conditioned to "work on the marriage". Women especially view a failed marriage as "their fault" and tend to stay longer than they should "for the sake of the children". As we mature, we begin to recognize that it isn't "us", it's simply that we made a bad choice. At that point, we may focus on a career or learn how to avoid confrontation (walk on eggshells), or we may turn to drinking, drugs, affairs, etc. to "escape" the person we live with...all of that to save ourselves.
Some women have a lower threshold than others for living with this. Some get out early and some (typically the ones who blame themselves) stay longer. Other's don't ever recognize they are living with a "monster" and remain for life. Bear in mind, the abuser is always manipulating this situation to create the "ideal" he desires. It is very difficult for anyone who hasn't lived through something like this to completely understand.