For sure, but we still have a massive proportion of what could be served by Mass Transit in the States all dedicated to cars, which is why gridlock is basically the American commuting experience in one word. I just spent a week in a major city visiting my employer's office for some face time with everyone, and every morning was started by spending about 20 minutes stuck in traffic, even though my hotel was only a short drive from HQ.
More than three quarters of Americans live in suburban or rural areas. If you tried to run an hourly bus down a suburban street in the middle of the day, it would have one passenger. You can't run bus service at the density of the suburbs, with single passengers it's just a slower, more expensive and less efficient Uber.
Then you get traffic congestion in the city because those people come into the city in their cars. But you can't just add mass transit where the traffic is because those people still need to traverse the part of their path where it isn't.
