I would say it’s very much about personal choices, but not 100%. Some of it is about environment. For example, if I wanted to become addicted to meth or heroin, I wouldn’t know where to begin.
My family has some issues with alcohol use and smoking-related illness, but no association at all with drug use, and that’s over several generations and an array of in-laws, cousins, etc.
Alcohol, weed and cigarettes are the issues in my family. Oh, and mental illness on my ex’ side. Like electroshock therapy mental illness.
I would say it’s very much about personal choices, but not 100%.
1. There used to be a very strong social stigma against making bad life choices. While those stigmas still exist, they have been substantially weakened over the last 60 years by a culture that has become very non-judgmental (unless you are a Trump supporter). If you remove those stigmas, then low IQ people (who have trouble seeing the consequences of their actions to begin with) are more likely to make choices that seem “good” in the short term, but are disastrous in the mid-long term.
2. As you noted, a lot of these family members never even graduated from high school. That in itself is a big strike against you. However, in the past, there used to be factory jobs for these people to take and still make a good living. Obviously, a lot of those factories have closed, moved or been substantially automated. Almost all factory jobs still in the US require technical skills and a HS diploma. Ditto with the Armed Forces.
3. For people without an education, drugs are unfortunately the shortest path to both relief and income.