That's fine for young childless people. For families with multiple children it's a huge hassle to have to have to shop like that. Growing kids eat a lot. And when you buy heavy items like watermelon or milk or whole frozen chickens it's just not easy to carry. Much easier to drive a big car to a big box store and stock up on everything needed for a week.

I know plenty of fairly affluent families with multiple children in European cities who never bothered buying a car. By the time the children were eating a lot, they were carrying part of the groceries. Starting around 10 or so, the parents would just give them some money and have them do some of the smaller daily groceries (bread, milk, fruit) on their own, walking or cycling.

You don't have to carry it, you can get one of those grocery carts. I see people bring them on my bus full of groceries.

plenty of people with young children in Europe and latin Americs manage to do just fine this way from what I've seen.