Why do parents abuse their kids?

When children are born in this world parents take care and love the child and help them find their way in life but not use their children as punching bags, when that child goes to school and sometimes get bad grades they show their report cards to their parents and when they show them the parents get angry at their child they kick their son or daughter in the stomach punch them in the face for no reason. Then when the child reach the age of the teenager they still get a beating every inch of their lives, but after words their parents say that they are sorry they will never do it again so that child forgives their mother and father, but they hit their children again and again thinking that the parents can do what they want to their child, and then they beat their child some more and says that their child diserves it then years later when a teenager reach adult age they try to forget their childhood lives but knowing that when you try to forget the past will haunt you for the rest of your life.