The simple answer is urban sprawl. Public transportation is widely used in New York, Washington D.C. and several other areas. In my city, my house is about 1 mile as the road goes and not as the crow flies, from the nearest major artery. There is no place to park a car and leave it while I take the bus to work.
I think it is more a issue of geography and desire. We have a bus system, it is under utilized and little public support for a tax to subsidize. However, for the poor and those who cannot drive, it is a lifeline.
Also, I agree with Larry Fields. We love our cars. But many cities are designed in such a way that walking to the grocery store is not practical, especially when the pharmacy is two miles in the other direction.