From my experience there are two factors I can see:
Due to end user advertising the US a lot of people are tuned into taking drugs. It's just a part of life. I am 50 and people seem to be baffled when I tell them I don't any prescription drugs unless I have something acute. I see tons of people taking antibiotics and pain killers immediately when they have a problem. That starts from childhood on.
Doctors in the US are really quick prescribing hardcore drugs. I have had several occasions when I had an injury like a bruised and got a Cocktail of vicodine and muscle relaxers. They make you feel real good quickly and I can totally see how people can get addicted. In addition people who have withdrawal symptoms get cut off cold turkey and get no help. I guess that's even more so in low income populations that barely get any healthcare.