[belval]

Probably because there is an upper limit to how much grocery you can haul back to your house on foot/bus. It's a completely different lifestyle.

Living in downtown Vancouver, I would go to the grocery store every 1-2 days to get food, because it was close by so there was no reason to "stock up". However growing up in the suburbs, we would go to the grocery store once a week to get all the food for the entire week.

A large scale grocery store/costco style simply doesn't make much sense as the place you go shopping every 1-2 days. So its better to put it in the suburbs and have people from downtown go out of the city every few weeks to stock up on specific items.