Why do girls prefer a guy who they want to fix?

I don't completely understand it still. I'm a nice guy and always have been. It seems as though from my experience with women, is that they want a guy who they can fix. They want a guy who is imperfect (jerk), so they can make him perfect (nice). Why? Why do they want a challenge like that? I preferebly don't want a woman who's white trash, has no respect, and treats others like crap. Why would anyone want to put up with all of that? This is what I don't understand about women. Why put yourself through a headache. It's like buying a used car. If you buy a used car, more than likely it will have problems and you would have to put money into it to fix it. It's better off to buy a brand new car with no problems. Why buy a headache? Same thing with people. Why date a jerk and put yourself through a headache? What are the ends of dating a jerk? To fix him and make him nice? What if he never becomes nice? Why not just go straight for the nice guys in the first place?