What would you consider an acceptable alternative? If criminals were not stripped of freedom of movement, then what would deter potential future criminals from committing crime? Prison is meant to act as a deterrent so that people with no moral compass have some incentive not to commit crimes.
Instead of focusing on the penal system, which is fairly hopeless, we should focus on preventing people from entering the penal system in the first place by focusing on systematic improvements that reduce criminal behavior as an appealing option. Look up "rational choice theory" with regards to criminal behavior: https://en.wikipedia.org/wiki/Rational_choice_theory_(crimin...
But that all said, the fact that the penal system is not mutated based on sensible performance metrics such as recidivism rate etc. is a human failing. The thing is, the system has already failed them at that point and they made their decision to not cooperate with society, so of course the people who have chosen to cooperate with society (the same people who have not sufficiently contributed to the systematic improvements that would have prevented that behavior to begin with) will fail to care about them.
My participation in this is to mentor young men (some are "at-risk", which basically means that their demographics are more likely to lead to criminal behavior), which I've done a few times and which is rewarding, because young men really need it right now. If I didn't have a 2 year old son who is keeping my hands quite full, I think I'd do it again.
The main purpose of prison is not rehabilitation, or deterrence, but to put dangerous individuals away so they cannot terrorize others. This is why we put rapists in cells (or, at least we try to) - every year in a jail cell is a year they aren't raping women.
the main purpose is punishment, see: nonviolent drug offenders in prison
one might argue a higher level purpose is to keep "the right people" in prison, where they lose their freedom, sometimes including the freedom to run against, campaign against, and even vote out the politicians keeping them there
