I'm coming to the belief that sometime this is an overblown excuse. I'm sure it's not true everywhere, but I just visited a friend in a medium sized (well under 100k people) Florida city, and spent a day going around by bus and foot, and it was great. Buses were reliable, air-conditioned, cheap, and traveled all the main routes, running about 18h a day, but they were barely used. Still loads of 6 lane roads, paved everything, massive parking lots, and more SUVs than I could count.
Even if you have a car for some trips, people in this city could easily reduce their usage, but they've become far too reliant on car culture. A trip to the store, 15 min walk, hop in the car. A trip downtown, 10min walk and 30min bus ride, nope... Car.
If we want more public transport, we need to encourage people use what exists when they can.