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.
Fresh food spoils quickly and often goes around the perimeter as a draw to get people to navigate the whole store.
Shelf stable and frozen foods last much longer, and are what they try to fill the middle of the store with. This can be deceptive in terms of feeling equivalent - all stores are going to have ketchup, but one may have room for two kinds and another has room for 20.