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.
The ones by me, ranging from cheap grocery stores, Walmart/Costco, through premium grocery stores, all have plenty of fresh food available.
They absolutely have aisles and aisles of frozen, packaged, etc; but outside of like specialty tropical fruits, there's nothing reasonable you could walk in for and not find fresh or at least frozen w/ minimal processing.
You can find dozens of varieties of fruits and vegetables, tons of fresh beef, pork, chicken and fish, milk, cheeses, even bread, in every major chain grocery store in the U.S.
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.
Where you shoppin’?
I live on the west coast in the US and the sheer variety of fresh produce would put any supermarket in the UK to shame, even Spain. California produces 40% of the nation's veggies and fruits.
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.
Your mileage varies, I guess. I used to live with easy walking distance of an upscale supermarket, but yet I did most of my shopping by driving to a different one farther away. Buying groceries with a car is simply more convenient.
Even after I moved out of that neighborhood, it wasn't unusual for me to stop at the grocery store every afternoon on my drive home.
Is that really how the average American shops though? The majority of shoppers these days are in the self checkout or "15 items or less" lines with only a single basket of stuff, at least in the stores I frequent. Granted, I'm close to a city center but the store I go to is not very walkable
Isn’t a monolithic place. I don’t think there is a non micro-state country in Europe where the absolute majority of people don’t commute by car.
Living outside of dense urban areas without a car is still generally tricky. In quite a few cities there are no large supermarkets in the densest parts and you have to drive further from the center to find one. So not having a car might be tricky
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.