Possibly because you can completely avoid alcohol but you can't completely avoid food. Some large percentage of people who would have been addicted to other substances (due to less than optimal nature/nurture) but didn't, because they did not take those substances, instead became addicted to the substance of food.
It might be more useful to see addicts to every type of substance as a large pool of anxious people who came across a substance to self-soothe, and figure out how to address the anxiety problem first.
The US population probably isn't much different from the rest of the world, in that regard. What's different about them is they live in the US. Drop foreigners from skinnier countries in the US, and, as a cohort, they'll gain plenty of weight (as with lifelong Americans, some individuals will buck the trend, of course)
It's a complex social-environmental, economic, and political problem.
"Don't commit violent crimes" doesn't work for most violent criminals, because they go back to committing violent crimes. Simple as.
"Stop gambling" doesn't work for compulsive gamblers, because they go back to gambling. Simple as.
"Have superhuman willpower in a ___domain where I don't feel temptation" isn't a cure, it's unempathetic judgment.