you miss the point. prisons are a massive industry with serious lobbying clout. Putting lots of people in prison for a long time isn't a punishment to the prisoner, it's a gift to the prison system. The "punishment" is just a side effect, and the longer it lasts, the richer the people in power get.
the harm inflicted by a crime has been detached from the relative prison sentence for a long time. Classic example is getting more time in jail (and loosing your house) for having a marajuana joint than if you raped someone. Fortunately this is changing in the case of marajuana but there are many other drugs for which a single personal dose will still get you more time than rape.