What happens to the inmates in prison that makes them worse than when they came in?
Small time crooks get to hang around with big time crooks. Its more of a training camp for scumbags than reform. Jails also boring full of stupid people who like doing things like picking fights or spreading crap because there is not much else to do.
And things like fights happen so someone might go in for a smaller crime, get in a fight defending themselves and kill the other person and they are now a murderer.
Its like sending a drug user to jail. Its a social issue and you turn it into a criminal one (I'm less sympathetic to drug dealers though). Jail is also expensive it costs the government/state $100k a year here per prisoner so its actually cheaper to send a drug user to rehab or counselling than prison- or pay them welfare. Even in the US its still 60k+ per year IIRC. You have to feed them, hire guards, run the prison, hire administrators and provide health care.
Some people need to go to jail obviously but you could probably release around half the prison population and redirect the resources locking them up and have it done cheaper and make the community safer long term.