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.
This is talked about if you follow urbanism communities. In addition to the reasons you mentioned, it just doesn't go to where people want to be. The last century of urban planning in the US has left transit and alternative modes of transportation as an afterthought or not thought of at all.
Land use is a major problem. In my particular city, half of the stations are surrounded by parking lots instead of actual destinations. Transit in the US has been treated as a band aid to car traffic, pollution, and costs. If it were funded and prioritized appropriately, we would see more transit oriented development and ridership.
Lack of ridership is seen as a reason to decrease funding. But when ridership increases, you get improved safety because there are more eyes to witness and report a crime.
I don't think most people make it moralistic crusade, but those kinds of comments and attitudes get the most attention. If you delve into the communities and read the relevant books, you may find that nuance is actually appreciated and discussed quite a bit.