Part of the problem is private prisons and their lobbyists.

A corporation's primary goal is to make money for it's share holders. Private prisons make more money if more people are in prison. So they lobby for harsher punishments, longer sentences, and against any plan that would lead to less prisoners.

It is in their best interests if inmates re-offend when they are released. We have incentivised not rehabilitating prisoners. Private prisons make more money when the offender comes out worse than when he arrived.

How is this good for society?