Nyororin,
Any statement that is a "generality" is just that, and you can find exceptions to any generality. Some US cities do have pretty good mass transit systems. And therre are places in Japan that have limited access to mass transit.
In each case you can certainly go to the extreme: You of course aren't going to find much mass trainsit in the middle of the Nevada desert. And you are also, of course, going to find good mass transit in Tokyo-to.
Overall however, I'd say that mass transit in Japan is FAR superior to anything the US has to offer. I've spent a lot of time in really rural Japan (I am a professional potter...... and have spent a lot of time in smaller pottery villages and such..... and out in the mountains of Chichibu, etc.) and have found better transportation offerings there than in the equivalent type areas of the US.
If maybe you take the US geographical physical equivalent to Japan...... which might be the eastern US coast from northern Maine down to the Florida keys...... (which might even be somewhat equivalent in population density too
)...... the mass transit availablity in the US and the mass transit in Japan are no where near the same.
I certainly totally agree that US people do not want to get around by any method other than a car. It is ingrained in them culturally from the time they are kids. Look at the brouhaha that is happening over the issues the US car industry is having.... which goes WAY beyond the significant economic issues. The car could be the symbol of the US. The US was built first by the railroads... and then by the car. Somehow we soon forgot the railroad.
best,
................john