As a family that tries to cook as many of our meals as possible and generally visits the store once a week, it can be a big endeavor. To make balanced, healthy meals for the whole week requires a large variety of ingredients. This makes meal planning before the trip and having a list necessary, and then there’s a lot of searching for things that may not always be in stock.

Additionally, we often restock household necessities and toiletries on this trip as well, which requires more planning and a longer list. Add all this together with carrying all the bags, putting things away, etc, and it can be pretty exhausting.

I'm not the person you replied to, but when I go grocery shopping I usually buy ~$200 worth of groceries and expend a bunch of energy hauling them all up 3 flights of stairs to my place which can be tiring, plus traffic to/from the store, plus putting them all away, clearing space in the fridge, etc.

I'd guess it's a combination of the physical and mental tolls of grocery shopping.

A lot of Americans don’t have grocery stores within 15 minute bike distance

Taking a look at the other replies can certainly explain how American grocery stores became such big places. I’m American, but have the luck of comparing to European town layout and actually being walking distance from a grocery store. Theoretically, if the shops (and streets and parking lots) themselves were smaller, you could make frequent backpack-sized trips for fresh whatever because it would be a tiny distance.