Sure. I mean, you can. I live in San Francisco and as often as not, walk to the grocery store. But it's a pain carrying bags around. Not everyone in the world is going to be ok with that when they are used to just packing a bunch of bags in a car. It's ridiculously unrealistic to think that everyone... including the elderly, etc ... will be happy to just use a train that might be a many block walk on each end.

Yes you can get groceries delivered fairly cheaply, but it will be cheaper with self driving vehicles and robot loading and delivery.

If going to get groceries only takes you a couple minutes each way then it becomes much more reasonable to carry a backpack full of fresh groceries rather than a car full of food that will become stale.

Density enables rapid transport as well as many more shops in a given area.

I see the elderly in LA carry groceries on the bus all the time. They use a grocery cart. Solves the problem.