The problem is that in a prison most of the people there are going to be criminals, and will end up socializing each other in ways that act against reform.
Though another part of the problem is that whenever you have a justice system anyone singled out by it as a criminal is going to have an easier time being trusted by criminals and a harder time being trusted by non-criminals in the future.
>>>and will end up socializing each other in ways that act against reform.
There's no incentive for people to reform which is a huge issue with the current prison culture. You're just trying to survive 24/7, align yourself with a gang for protection and hope you can make it without getting caught or extending your sentence.
The whole system is set up for people to go to jail and get better at committing crimes.
The prison gang system is also supported by (some of) the warders, because it makes their job easier. Not just because some of them may receive bribes, but also because they come to rely on gang authority to keep any kind of order.
Though another part of the problem is that whenever you have a justice system anyone singled out by it as a criminal is going to have an easier time being trusted by criminals and a harder time being trusted by non-criminals in the future.