Just how I grew up. I'll use grams and pounds for 99% of things, and rarely see KG being used for weighing people or animals around here.
Many store items are also mixed units... potatoes are in lbs, frozen vegetables in KG, cereal is in grams while hardwood pellets are in lbs. One will use oz while the other will use ml.
I don't care, as long as I understand what the hell I'm getting. 😂
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.
I mean, one large watermelon and a 10lb bag of potatoes already gets up over 20lbs. Add in a ton of bananas (to freeze for making ice cream), 5L bottle of vegetable oil, 4 x 1.89L of almond milk, and other food, and you get to 129lbs 😂