Walking distance covers a large range. There is a big difference from a place you are walking by anyway, and a place you can walk to but normally don't. I can walk to my local grocery store - it is only about half an hour: about as far as a normal person would walk to work. There is nothing else along that trip that I would go to regularly which means that grocery shopping by walking would cost me an extra hour of my life every trip - in turn meaning I'm going to buy as much as I can. Anybody with a family will be buying more than they can realistically get in a cart (assuming any hills or rough terrain)

A suburban grocery store is rarely much closer - even though there is typically houses behind them, there is a fence in between so you can't get there from those houses.

If the grocery store was on my way home from work that would be different: I can stop in to grab just enough for today's meal (and breakfast tomorrow). However on the way needs to be fairly strict. A suburban grocery store has the doors too far away from anyplace somebody would walk to be considered on the way - the additional to my trip is going to be annoying enough that I'm going to be getting more stuff when I do make that trip in turn meaning I won't use it.

The above assumes a pure walking trip. I used to take the bus to work. The bus stopped at a transfer point in front of a grocery store, the bus only ran every half an hour so I never got off for my shopping: it didn't work for a 5 minutes get what I need for today trip as the bus would be gone (5 minutes assumes I know exactly where the one thing I need is and that the line to checkout isn't long). I'd either have to waste 20 minutes, or I'd get so much stuff it can't fit on the bus - also there is risk of a long line making me miss the next bus.