A huge part of justice system is exactly that, justice for the victims. They want the offender to be punished and pay for harm they caused. It would be unfair to the victims if govt was easy on the offenders.
Edit:
Depends on which end of spectrum you think humans are responsible for their own actions. Between,
1. People are merely the product of a zeitgeist.
2. People are 100% responsible for their own actions.
Most people think human nature is somewhere in between and our justice system reflects that thinking. Any rant saying only 1 is the right way to think about justice system has never been personally victimized or brutalized and would be singing a different tune if their loved one was a victim of some senseless/sadistic behaviour by another human being. People who live in crime free neighborhoods love to theorize about how we can be so kind of criminals and that will bring the best out of human beings.