I think growing out of it is a big part, though. I knew a number of people who shot up in their late teens/early 20s who quit when they got older. The methheads I knew had a harder time, but a lot of them quit too. Of the people I knew who were hardcore drug abusers, very few died, and most grew out of it, which is what the numbers suggest. The ones who and the worst time of it were the ones who bought into NA/CA dogma, and took on addiction as a personality, rather than seeing it as a phase they grew out of. They also had the highest mortality rate, predictably. Tell someone they have no self control, and they'll binge the worst.

Some people who get hooked by drugs die, but addiction isn't nearly as deadly as it would need to be for the numbers to drop like they do, and statistically spontaneous remission is the most common way for people to kick the habit. Normally people grow out of it.