Is there no contrarian position on this? Very rarely does the general stance on an issue seem so unanimous. Surely there must be some defense.

I've always thought it has a lot to do with them having completely different goals at the fundamental level. It's why you can privatise things like railroads and postal services, as both the public and private sector have the same objective (run a good, efficient program trying to expand and get more people to use the service). But private prisons are not like that, as they can only really expand and turn higher profits by being paid more by the government (which is already a no-go because of how costly that is) or by dealing with more prisoners. On the other hand, society as a whole and the public sector want fewer prisons, less convicts and less crime, which opens up a whole lot of possibilities of abuse by the private sector.