Buying an entire cart full of groceries that will last for weeks is a somewhat American cultural thing. I'm not saying that I've never seen it, but the norm where I live in Europe is to have one of those hand-held baskets and getting enough for 2-3 days tops.

Or they just don't live in cities which not everyone in Europe does either so they can't just walk a few blocks to grab supplies for a day or two.

Definitely an American thing.