At some point during their rock bottom period they realize the sad truth about life and how nothing is real, nothing matters, etc. Dumb addicts feel it instinctively, smart addicts have it surface to a conscious level.
In rehab there is this whole thing about finding a higher power. I assume this is like finding something you'll decide to believe is meaningful so that you'll have something to grasp at even when you're going down a quicksand. Is it much like existentialism, the path they take back to humanity? They agree to resort to delusions in order to recover to functional levels and go back to the conveyor belt with the rest of the monkeys? They may have lost a great portion of their lives and have few pieces to pick up. By the time some of them recover, they are past their prime.