Here’s a better theory: because American public transit is, when compared with the alternatives, not safe, not clean, and not convenient. Take LA, probably the most car-dependent big city in America. Riding the bus or subway in LA is not an enjoyable experience. Nor is it enjoyable to walk around the areas where the stops are. If I were trying to get more people to use public transit, I’d start by making the stations and buses/subways beautiful, clean, safe places that are just nice urban places to hang out in. There’s no need to make it a moral crusade; just offer a better product and more people will use it.
Seeing tons of videos online of interactions on the New York subway system, I can say that I have no interest in that form of transportation. The recent drama about Penny/Neely is just one of many such interactions you can find on the subway. I can link dozens of videos of insane, disturbing interactions that take place on the NY subway to which I would never subject my family.
If we somehow create subways that are as clean, safe, and convenient as those in Japan I would probably consider using it, but until then I will definitely be pro-car.