Conker you obviously don't understand the nature of abuse. If abusers were abusive from the start of a relationship, nobody would stay with them, so they put on a front, and act all nice and caring. It's not until their partner has fallen in love, moved in, etc, that the abuse starts, and it usually starts off gently, slowly, pushing boundaries.
Every now and again, the abuser will act like they did at the beginning, to confuse their partner, and it makes them think "He's got a nice side, so if I just walk on eggshells and don't do anything to set him off, we'll get back on track", and they stay.
Buying a house/moving in together, getting married, and/or having a baby, are the most common triggers for abuse to start. These trap the abused partner, many of whom won't have the finances to move out, because they're SAHMs. Some do try, but the abuser threatens to take them to court for custody of the children, and because the abused partner is so confused, they believe it, and don't want to risk losing their children.
There are lots of ways women are abused, and the reasons they don't leave, are complex. Some never leave, others often take several attempts to leave, before finally escaping. I suggest you educate yourself about it, because even if you never experience it, someone you know may, and you could give them really bad advice because of your ignorance. The reason being, what would be advised in a marriage that's just going through a rough patch, such as "talking things through", doesn't work in abuse cases, and can lead to the abuse escalating.