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.
In general, a city is more walkable and dense the earlier it developed. NYC and Boston are walkable cos they're old. Parts of Chicago are, but it did most of it's growing post-car so most of it isn't. LA did practically almost all it's growing post-car and so is awful for walkers.
It's the same in Europe - most of London is walkable because it hit a multi-million population pre-car. Milton Keynes is a concrete car-jungle because it only developed post-war.
All smelled of fresh paint and wet concrete. All were built with the intent to be walkable, and all are wonderful places to live. I never felt the need for a car once. What matters is not the age but the intent of the designers.