Why do parents starve their children?

I watched an episode of Dr. Phil that showed of an eighteen year old discribing her abuse, that she was locked up in a closet by her parents she was chewing on bowls,biting wood from the door, laying on her feces and other things i rather not discribe. The neighbers tried to save that child including the police but  what was worse the parents sexully abused her and beat her like a punching bag around the clock the neighbers were not happy for what the parents was doing to the child, when the police arrested the parents the good people saved that child from abusive parents and that child was put into a good and caring home, but when the mother was brought to court to be put on trial for child abuse the mother did even shed a single tear nor did she say that she was sorry for what she did to her daughter, people would say that she was proud of what she did to that girl. A child should not be abused nor totured in a home of abusive parents that child diserves the right to be loved and cared by a loving family but not raised by a cruel family.