The American diet is insane, full stop. However, I've just begun a GLP-1 regimen to address a willpower problem, not a nutritional problem. I'm not quite young anymore and have given lots of other approaches a shot over the years, but have persistently failed to achieve a weight that is not a threat to my health.
So far, what being on a GLP-1 gives me is a steady state that most people probably find quite unremarkable: I don't crave a snack, and I don't thirst for alcohol. Both of those desires have had real control over me for a very long time.
Versus the UK, any US supermarket I've ever visited (I lived there for a couple of years) seemed to have far less fresh food, especially vegetables and fruit, but stuff in boxes was piled high.
Then again, the UK vs. Spain or France is weird, by the same metric, they have even more fresh food than us in supermarkets, and much less boxed stuff.
Geography and having continent sized country probably doesn't help either.
Some Americans are surprised to learn that many supermarkets inside cities do not even provide parking, everyone walks or bikes there. People go to the supermarket every day.
When I lived in Europe for a couple months, my first time there I grocery shopped like an American - filled up an entire cart with a week or two worth of groceries and then everyone stared at me when I checked out.
It's absolutely true that Europeans who live in walkable cities go to the market to pick up groceries a few times a week. Americans simply do not, with very few exceptions.
The grocery store density is much higher though. There were at least 2 grocery stores within a 5 minute walk from anywhere I've stayed in a city core in Europe. At least a dozen within 15 minutes.
It's simply a difference in culture. There are plenty of places in the US where you could drive to half a dozen grocery stores within 15 minutes but people simply don't do so. The store sizes reflect this cultural difference too. The average grocery store in the US seems to be 4-6x larger than those in Europe.