Because they love their parents and keep hoping things will change. And abusive parents, like abusive husbands, have a talent for making the victim feel like they deserve the abuse. Then the kids grow up learning that is the way to "discipline" children and how to twist it around against their own kids.
I don't think children understand abuse until the age of true reason, around 12 is that time in growing. They don't know what to do before that time, except hurt. I think we should be more aggressive in teaching our kids about abuse and what to do about it, if themselves or others.