They are mutually exclusive in the US because space for cars (current US sized cars) means everything is farther apart, which means the public transit is not economical, or lots of walking. And walking is more dangerous for pedestrians due to inattentive drivers and large arterial roads with wide crossings.

The constraints lead to completely opposite designs, which is why only very few, very dense cities in the world have convenient public transit, and they also happen to be inconvenient for personal cars.