This depends on the country a bit, but in general, prisons are the opposite of rehabilitation, despite the countries running them telling the contrary.

How gets one rehabilitated when they are pulled from their home, their job, their social surroundings? Supposedly, people are rehabilitated into society - the one they are excluded from and have no access to in prison. Especially in longer sentences - no internet access, years of societal and technical disruption, societies change in the absence of the prisoners. When they get released, they usually have to start over but with the worst conditions - while rehabilitation in prisons claim to help them go l with their standing.

Rehabilitation taken serious would look completely different. Prisons are preventive custody and everything else is an excuse to distract from that fact. Some people may try their best in helping prisoners, but they are working against a system that destroys the chances instead of rehabilitation.