>seniors become homebound and isolated sooner when they're no longer able to drive themselves to the grocery store

large supermarkets (naturally can't be just in every small neighborhood) -> efficiency and lower costs. What is the point of local (thus small and high priced) store that a senior can walk to, yet can't afford to shop at?

Small stores can be perfectly affordable. National Co+op Grocers lets local coops buy from UNFI at a national bulk rate, just like the giant chains can. I shop at a member store and usually pay less than I would at one of the big chains.

This is simply your gut feeling, smaller (and cheap) supermarkets are common basically everywhere outside North America. Chile which is in no way rich or some urbanist paradise is a specific counterexample, prices only go up with specialist stores (e.g Whole Foods equivalents) or small business of the size of a parking spot in someones home, think like NYC bodegas.

Cars are expensive, the total domination of personal cars is only possible if you are a rich country. Much like gout.

You are partly correct to note that local markets are often not economical, but the reasons for that are precisely low density, car dependence and restrictive zoning.

For a local market to be economical it needs a catchment area containing enough customers.

Once customers start driving they prefer to drive a bit longer to go to a mega market or mall with good parking options, i.e. not a local market.

Local markets thus have a catchment area that is defined mostly by walking and biking.

(Not having a lot of parking can also help to make local markets more economical)

Some important conditions for local markets are then:

1. Zoning needs to allow the shop to be very close to residential areas, and with only few parking spaces.

2. Residential density needs to be high enough to have a large number of customers within walking or biking distance.

3. Walking and biking should be sufficiently safe and pleasant.

It is easy to see the why there are few local markets in suburban North America, but they are very common in urban places all over the world, as well as in thoughtfully designed and slightly denser suburbs in many countries.