I think lack of brain development has a lot to do with it too.
In the family program I took (run by my sister's clinic) they told us that when a person starts taking a substance to numb feelings or avoid unpleasant thoughts their development pretty much freezes there. They never do the hard work the rest of us did to mature.
So when they stop using, you are still left with the same maturity level they had when they started. In my sister's case, still a teenager going on 30.