Eh, this is just not proven true by observing what people do though.
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.