I think your main point is right, but there are many Costco members who are NOT shopping for "weeks of groceries" at a time, and many of them live in suburban or urban areas with high density. For example, I shop at Costco once a week for just my girlfriend and I; we don't buy outrageous quantities. We live in a populous area.

Our situation is pretty common; it's just a normal grocery store in effect for lots of people. The weird stereotype of Costco shoppers driving for miles to buy huge carts of food just doesn't line up with the typical case for my area.