Subways are pretty much exclusively built in the cities
Not just any city. Dense cities. Cities that are so densely populated that it would be /impossible/ for every person to move around in a car. Countless US cities are not even close to crossing that threshold. It just makes no sense to look at nationwide per capita on this. Only a city by city comparison of like with like population density is sensible.
(edit)
There is a baby elephant in the room that needs mention: US cities are designed with shitty zoning plans. They are designed so that each person on avg needs to travel more distance per commute to accomplish the same tasks (work and groceries). This heightens the congestion per capita. So ideally we would calculate daily net commute distance needed per capita plotted against subway track per capita for cities of comparable people per m². Which would embarrass US city mayors even more.