European cities started on a scale designed around the limitations of human walking. Even before they built out mass transit systems, living without a car was doable. Adding mass transit is icing on the cake. If you live in such an environment, it is easy to make the case that cars are a luxury. That's because other people's daily experience is that it is a luxury. As your mother discovered when she visited you.
Most American cities are built on a scale designed around human driving. Even where they built out mass transit systems, between scale and density they can't work as well as European ones. (Fun fact. Across the USA, busses are on average so underutilized that we'd save gas by making everyone drive instead.) Underinvestment in mass transit is icing on the cake.
This was your mother's daily experience in suburban Massachusetts. And even though she sees how you can live without a car, it's pretty safe to bet that she doesn't think that she can live where she does without a car.
Which means that, in America, saying that cars are luxuries is a poor argument. It directly contradicts everyone's personal experience. Yes, this is fixable. But fixing it literally requires tearing cities down, then rebuilding them on a scale where walking makes sense as a major mode of transportation. We can't even manage the political will to build enough housing to keep people off the streets. Any lifestyle change requiring this level of rebuilding is a nonstarter. No matter how many lectures we get from Europeans.