Its not just prison, it's the entire perception of crime and punishment in the US.
Its why convicts cannot vote in elections even when they get released.
Its why they still have the death penalty. And why victims families can watch them being executed. Its eye for an eye, punishment and not forgiveness and rehabilitation. Its much deeper than the quality of life behind bars.
The system is also exceptionally biased (certainly so for a democratic country where ostensibly people are free, equal, and corruption is low) against the lower income layers of the society and based on race, while the top layer of income/influence gets a wildly, ridiculously different experience. [0][1][more]
The US doesn't seem to have a justice and penal system, they have a modern slavery system.
Felons not being able to vote is not universal. It is up to each state to determine whether to allow felons to vote. In recent years restoration of voting rights to felons has been on the rise.
American prisons are a shame and there is absolutely no way that convicts come out of it ready to participate to society.
This youtube channel gives a good perspective on how inhumane life is in American prisons: https://www.youtube.com/channel/UCzmkeda2XiYpETOP5MjotrQ