The first problem is that it isn't correctional. Secondly, it isn't preventive. Third, it forms its own society. Fourth, it has become "cruel and unusual punishment." Fifth, it is overcrowded. Sixth, the guards are not trained in psychology or sociology (most do not have a college education). Seventh, it is underfunded.
1. Virtually no effort is spent in rehabilitation, except for 12-step programs and the like. No-one ever discusses "correcting" their mistakes.
2. Wrong-doers virtually always report that the consequences of their crime did not enter into their decision.
3. The in-prison social set-up, based on violence and threats of violence, run by gangs and separated by race, is completely out of hand, leading to
4. The Constitution EXPECTED that taking some years out of one's life would be the punishment, not rape, stabbings, intimidation, etc.
5. Hundreds of prisoners are stacked up in large rooms, because there aren't enough cells.
6. The guards are not allowed to treat prisoners like individuals.
7. No money is available to change any of the above.