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.
*I say "perceived safety," because vibes seem to matter more than actual safety. Like, the stats on car wrecks, drunk driving, distracted driving, and so on are alarming. But when I think of someone concerned about "safety," I imagine someone being uncomfortable around people they feel are sketchy.
Convenience is a big part of it, sure, but even Americans will use transit when it works for them, even if it is not faster (it is almost NEVER faster than driving a car unless you do strange restrictions or include a very-high-speed segment).
But you only need a few bad experiences on transit to put you off it when you have other options.