Life isn't fair, but do you really want to walk around with people like that out in the world?
Society is grim and unjust but the reality is the best that can be done is to remove these people from society.
The world revolves around money so the paradigm is geared towards self preservation, theres no money to be made from stopping somebody from becoming mentally ill. The money is in the treatment. And broadly speaking, society is reactive not proactive. I doubt these things are going to change in our lifetime.