First, I wouldn't call hem private prisons, I"d call it outsourced prisons. Government still retains a complete monopoly on every aspect of the judicial system and the prisons must be built how the government tells them. Second they are like 10% of prisons so they are not huge deal and certainly didn't cause any problems with our prison system, they've been there for decades.

As for lobbying for new laws and such, that's true, and it's typical government to create a way for people to profit off the harm of others. But again they are only like 10% while 100% of the prisons are filled with union guards who lobby a lot more for more laws along with the police unions.