Because people buy much larger cars than they need, particularly SUVs.

Because so many people live a good ways from their work and prefer to commute in their own cars, even if they have to take an hour for what should be a 20-minute trip because they spend the other 40 minutes stuck in traffic jams. This is aggravated by restrictions on smoking, since the only chance many people who are addicted to nicotine have to smoke during the day, without freezing their butts off, if they live in a cold climate, is in their cars. This in turn restricts car pooling, because of the conflict between people who want to smoke in their cars and those who can't stand it. (Mind you, I hate smoking and think the restrictions now in force are great - but you have to consider the side effects.)

Because so many people would rather commute in their own cars, as noted above, the demand for good reliable public transportation is not large enough to support it, so additional numbers of commuters have to use cars, even though they'd rather use public transportation.

Add to the above, the fact that the government (I'm talking US here, understand) gives massive subsidies to the airlines, and provides the airport facilities, and essentially underwrites the trucking and bus industries by providing the highways, but feels that the railroads should be self-supporting, thereby making passenger rail non-competitive.

Because so many perfectly healthy and able people's idea of getting outdoors for recreation is to go for a drive, or a motor cycle ride, or play golf using a motorized cart, or ski using a lift, or hunt deer by driving the truck around the back roads with a rifle sticking out the window (It really happens - come to Vermont in November if you think I'm kidding!) rather than walk/hike/run/bicycle ride/cross country ski/skateboard/whatever.

I could go on, but I'm likely to slip into blathering. Maybe I did already.

Peter.