As an outsider (most people in my country don't shoot guns for fun, but we still have our fair share of morons) I think not educating oneself/not being educated may be a more important cause.
My personal opinion is that it's more related to the way people spend (waste) their time. All of us, I mean. The way we (do not) educate ourselves, the way we do (not) value intelligence and knowledge.
- How many people shoot guns? vs How many don't ever read a book (a difficult one, I mean, say one essay a year)? or How many students reach university level without having read a single book? FFS, if that doesn't ring an alarm bell...
- How many people are (not) being taught how to have heated but articulated discussions, in the literal sense of debating against someone, having a dispute with someone, while still being able to not want to kill one another?
- How many people are willing to be told (and willing to admit that) they were wrong... when they were?
That lack of education and an overall cheerful ignorance of all facts that dare not fit their viewpoint, no matter which one it is, seems to me a much more likely cause to explain why more and more people around the world (not just Americans) 'seem cognitively impaired'. And that's because, well, they are. Sadly.
We don't value knowledge anymore, we value money and success. Once again, suffice to ask people: how many essays did you read in the last 12 months? Or to look at kids, how many of them want to be, say, a doctor, a scientist of some sort or, even funnier, a writer? And how many want to become 'an influencer' on YT (or TikTok, or whatever) or to become some star singer or sport star?
Kids have not suddenly become allergic to smartness. They're only the mirror of what our real values as a society are (not the ones we pretend to have). Which are not being smart, not even talented as a matter of fact. They are: easy money and success.
imho, this is the main cause of dumbification going on everywhere. Obviously, I may be wrong and maybe I should stop eating lead bars as a snack?