That's really it. Rehabilitation and retribution are only minor factors, for the liberal and conservative voters respectively. The main factor is just stopping crime by warehousing people until they die or get too old to cause trouble.
It's expensive, but you have to admit that it is extremely effective.
I've done time in the federal system, and on one of my transfers between prisons, I got to sit beside a supermax transfer prisoner. Really, he was a pretty nice fellow. Said he had to kill a guard once in a while to get out and see the sights and have some decent food. He was on his way to court for that murder case trial.
The explanation that some people just ignore the laws and think they can get away with it is too simple, isn't it?
>> Humans are not perfect reasoning machines
Neider are they "stupid saints". They sometimes just take a huge risk for whatever reason. And sometimes this risk involves getting jailed, because society consider some kind of risks as undesired and make you pay a high price for taking them.
A hint: lot of the "incredibly stupid" things are being done in the heat of the moment. Other times, people don't really imagine something bad could happen to them. Then, sometimes people think they have no other choice - maybe they're "in too deep", or maybe it's the only way they can think of supporting their family, etc. Sometimes people just take a huge risk for potential huge gains (think of it as criminal world equivalent of starting a startup).
Humans are not perfect reasoning machines with infinite computation speed.