A lot of people, regardless of political persuasion, believe that the purpose of prisons is to punish those inside. No more, no less, and any "rehabilitation" is just something that happens alongside (and often in spite of) the real purpose.
The (US) Constitution prohibits "cruel and unusual" punishments in sentencing, but in my mind, losing years of one's life, which one will never get back, is significantly more cruel than, say, fifty lashes.
SCOTUS has said about the death penalty specifically that it is without a doubt cruel; however, it is pervasive, and therefore not an unusual punishment. Therefore there is no Constitutional violation. I suspect a similar argument in the inverse rules out corporal punishment. Fifty lashes, even if less cruel than decades in prison, is certainly a cruel punishment. It's also very unusual for 2017 America.
Prisons are a dumping ground for people with personality disorders. You can't fix a personality disorder with rehabilitation, or with punishment. (Arguably you can't fix a personality disorder at all.)
Some people are relatively rational. They commit crimes for a variety of reasons, a good proportion of which are a direct outcome of poverty and/or economic abuse and/or racism.
The best way to rehabilitate them is to rehabilitate society so it doesn't push them into a corner, or create subcultures where crime is a reasonable survival option.
The people with psychological issues are much harder to deal with. Keeping them locked and away and occupied is more humane than trying to "punish" them, because the punishment is pointless and will have exactly zero effect.
Running prisons as a for-profit industry instead of a social service is barbarism, and ultimately just puts more criminals on the streets.
If punishment is rehabilitating, in average, the harder the prisons the nicer the people that come from them. We could check for instance, I don't know, Russia vs. Sweden or something like that.