The abuse of the scapegoat child is so deceptive and manipulative and so complex and in depth and the abusive covertly malignant parents are so obsessively dedicated to playing the innocent parents who are “just trying to help their troubled child”, that the victim often sounds “crazy” when they try to tell people what happened because it is just so incredibly hard to believe. This is compounded with the victims complex PTSD that they most assuredly have after years of this abuse. 
And the abusive parents count on this. It’s like a game to them; make their victim look crazy.
“But why would anyone do this to their own children?”
Why do psychopaths kill people? Why do sadists hurt people? The problem is that people don’t realize how common and covert these abusive people are all around us. Many people are friends with people like this and have been friends with them for years or decades and they are just sure that the victim must be delusional because they can’t believe it to be true. And this is so common it’s frightening.