I don't know what grocery stores you go to, but I've never seen someone standing at the checkout line, holding a single turnip.

Corner stores have the occasional sub-$5 purchase, as do, of course, coffee shops.

Single turnip is ridiculous sure but that is hardly the average sub 5 $ purchase. I have seen people check out with single pieces of fruit or like a gallon of milk regularly. Plus, in my state, you could easily get a single deli item for lunch or breakfast for ~5$ or under. Basket size (which is the term groceries use for the average purchase total) is about 50$ although it has gone up in covid19 times. That is the mean though- there is a ton of ~5$ and sub purchases

I work near a grocery store and quite a few people by a salad or a small snack for lunch. Some also quickly go and some small things that have run out, i.e. some milk, before heading home.

This might also be a currency difference, through; 5 Euro in Germany can get you quite far (at least outside of metropolitan areas).

I have seen people purchase a single Brussels sprout in order to use a coupon that requires 'any purchase' to get a free item. Getting two of the same item is more common, but having someone go back to find the smallest vegetable that exceeds the tare isn't unknown.

I go to a major grocery store every day and buy a single diet coke. Self-checkout changed things a bit (less lines, generally).