Yeah, so the fuck what? Government prisons are incentivized to be even worse, and private prisons have competition.

A stateless society might find a use for something like what certain private prisons do, which is basically rehabilitation and jobs for people who owe a debt to someone they've wronged (in statist society they owe it according to a monopolistic judiciary to the government, and the government pays them, in stateless society this might be funded a number of different ways and could be useful to give the riff Raff something to do to better themselves and pay off legal debts). There would be market forces determining which prisons were patronized, and they would disappear altogether when not needed, but they would exist when it is efficient for them to exist. You never know dude.