It is to a point, but when you reach that point it's just not feasible to have public transportation. The city I work in has a light rail train, it has a robust busing system, but people also travel from 20+ different small towns around this one and at a certain point that system breaks down. If I were to take the bus I would still need a car to get to the next small town where the bus stop into the large town is.

The fact is that the most of the US isn't designed like old world cities which were built with public transportation and foot traffic in mind.