The reason prisons are not very effective at preventing and deterring crime is that prisoners have lots of time on their hands to learn even more crime from other prisoners.
Some prisoners don't care about being in prison and will commit crime again and again even after they get out and go back in.
Prisons do not really do any correctional things to the prisoners and although some prisoners learn to become good people some do not.
Prisons are not effective in preventing crime.
Although some people fear the possibility of prison or jail and that is enough to deter some people from committing crime and some people who commit crimes for the first time and go to jail may decide it's not worth it.
That can sometimes be enough for some people to avoid getting into trouble.
However some prisoners who are in prison for a long time and then released may actually commit more crime after they are released.
There have been prisoners who have served 20 years or longer in prison for robbing a bank and then they got out and then committed another robbery within a few days or so.
Then they got back in prison for another 20 years or so.
Being around other criminals in prison actually makes it more likely for the prisoner to commit another crime of the same crime or worse when they get out.
We do need prison systems though for really dangerous people that would be bad for society.
But prisons do not rehabilitate people or correct their behavior like the name correctional facility insists.