That's the problem right there. We can spend tens, hundreds of thousands of dollars incarcerating a simple marijuana dealer, and then remove most of them from the chance of ever having a productive life.
There are exceptions. I have an old friend who spent a few years in prison for LSD traffic back in college. He's one of the smartest people I ever met. Eventually he got out, finished his PhD in chemistry, and now is the co-founder of a startup doing new chemotherapy drugs. But like I said, one of the smartest people I've ever met.