Speaking only for myself, a large haul like this happens maybe once a month. I usually buy for more than the week (or the next one), so a large bag of rice, for example, will last quite a while but will take up room/weight for that grocery run.

For grocery runs in-between the large hauls, which are mostly perishable goods, I can often get by with just a single pannier, or maybe two if I'm getting something that takes up more volume.

The less often I'm in a store, the less I tend to spend throughout the month, so it's a strategy I use to keep my grocery budget in check.

I'm guessing we just don't go shopping as frequently. We usually go to the store every other week or so, so we buy 1-2 weeks worth of groceries at a time. To get bulk discounts, we tend to buy something large each trip (25lb bag of flour or rice, 20+ rolls of toilet paper, etc).

That said, I've never had a problem doing a trip on my bike that can only support 100lbs (kid trailer), and I find myself going more frequently with my bike because getting to the store is more enjoyable.