Even the places that have public transportation here, don't have public transportation. My city (not big enough for a subway but on the larger side as far as medium-sized cities go) theoretically has a bus system:
As you can see, there are large sections of the city where the busses don't go at all. Even if your home, work, grocery store, doctor, etc. are all within walking distance of a bus line, you're looking at up to 30 minutes of waiting at a bus stop (and that's if the busses are all running equally spaced and on time), plus walking time to get there, plus the extra time it takes to ride the route (and transfer to a different line, potentially) over what it would take if you owned a car, including all the extra stops and distance it goes. Sure, fare is a dollar, but you're giving up two hours of your day just to get to the store (and that doesn't include the time you're actually shopping).
When I was a librarian, my branch was in a little town about 20 minutes away from the city. Median income was right at the poverty line (~$15,000/year). There were a significant number of adults in that town who *couldn't* work - they couldn't afford a car, there was no transportation to anywhere they could get jobs, and the one tiny grocery store in town only needed two part-time employees. It was really a waste of manpower, because all those people were forced to waste their days watching TV and collecting meager government checks when they could easily have been earning something more substantial and really contributing to something