Building a tunnel and then filling it with cars is a waste of space. The per capita gains don't just come from being underground, they come from density of people - which is abysmal for cars, multiplied by the fact that most people drive alone for most of the time.

Mass transit rail has hardly "struggled" in the US - it has high usage and conflating the distinction between mass transit rail and long distance rail is disingenuous when considering the usecases of Musks tunnels.