I suspect that arrogance is more at fault than stupidity. Arrogance leads them to believe that they can experiment with meth, heroin, and other addictive drugs, and not become addicted themselves. Then, after they become addicted, they deny their addiction, imagining that they can quit anytime. From there, they perform so poorly at work, get canned, start stealing, and worse, to support their habit.
The few ex-addicts that I've known, whom I believe will never fall back into that trap, first had to kick their arrogance habit, to be able to kick their drug habit.