People don't like to be told the truth.That's the pretense, the real reason is Americans all have this inane notion that they will all be billionaires or at the very least multi-millionaire and when that happens they won't have any need for subways.
The rich are the demigods that the American masses worship and want to join their club one day, until that time they need to put in their due, i.e. suffer while grinding out 3 jobs without any support from the gov't and paying exorbitant tax relative to their income so that the rich can trickle down manna on them.
The truth is that if you are both not rich and living in a city, you are highly unlikely to generate the wealth that you need to sustain said city.
This is totally unsustainable, and therefore eventually is not sustained.
This is the real reason why most Americans do not live in cities.