That's a very good question - how many people are "found guilty" each year in other places? If they just kill them off then it might seem like they have far less "bad people" long term ...
And yes prison seems like anti-rehabilitation in many ways because it puts the offenders in with other offenders where they can learn bad skills, traits, personalities, etc. I would be *terrified* if I was sent to prison. I would probably hide in a corner and have a breakdown.