I don't know the medical/psychological answer to this, but my gut feeling is that addicts/alcoholics have underlying emotional, personality and/or behavior issues that they used drugs/alcohol to deal with. They never learned to deal with things any other way, and so when they get clean, the problems are still there.
I think it is a common misconception, mostly from folks who mean well but just don't know what they are talking about, that all the problems will be gone if the person just gets clean. I have experienced this attitude from folks in my church that are trying to 'help' my AH.
I agree with several things cmc said; I think that addicts get emotionally stunted because they never learn how to cope except with a substance. I also think that, for lots of them (including my AH), the mess they have made of their lives after years of using is so deep and so wide that they are totally overwhelmed by it, and without something to numb reality they can't deal with it.
There is also an issue with real physiological damage to the brain after years of abuse.
Just my thoughts. Interesting thread! I look forward to other responses.