I think it's called a life sentence because your life ends more or less when you're in prison. Prison does not rectify someone's character - it just punishes people.
If you're lucky to survive prison and come out your life is still more or less over. People who have done serious time will tell you that when they come out they have a terrible time fitting in the regular flow of society. As a famous quote says:
"I went in with a bachelor of marijuana and came out with a doctorate in cocaine."
Prisons are detrimental the way they are now. They do not solve the problems of messed-up people.