I've thought about it quite a bit. There are exactly 2 reasons.
1. Americans invented the "Hip" and "Square" concept. Whereas Euro-pee-ons have all kinds of deeply ritualized mechanisms by which they determine their pecking order, usually involving being the most "cultured" and most "the way you are 'supposed' to be", Americans create the social popularity ladder by being most interesting/individual/funny/or entertaining. Flashy cars, expecially unique or "other than normal" work well to that end.
2. America grew up and continues to be COMPLETELY dependant on cars. The car is a way of life because the car is necessary for life. Minneapolis got its FIRST light rail train just a few years ago. A Japanese friend expressed astonishment when I told her about it, I answered (she didn't own a car, neither did her family). I explained if I didn't have a car I'd starve to death and die in addition to not be able to go anywhere I want to go. I'm not kidding, my car broke down so I was without for about 4 days once, I got hungry (not much of a cook, nor did I have food in the house to cook). It isn't that Americans are obsessed with cars, its the Europeans can imagine cars to be an optional part of their lives. For Americans, they are not.