1. Public transit is very slow relative to cars. This is because the U.S. was mostly built after the invention of the car, which means road capacity is higher, parking is easier, and distances are farther. Even in places with decent public transit, it takes 2-3x as long to get somewhere on public transit as driving a car.
2. The U.S. is a rich country and more people can afford their own car or to take a taxi/rideshare.
3. Public transit is less comfortable and convenient. It involves walking and waiting outside, possibly in bad weather, and possibly crowding. If you are carrying things with you or bringing small children, the hassle is magnified trying to maneuver through small spaces with large items quickly.