America has really shitty public transit, so on average, you're driving (or being driven) somewhere and seeing other people driving.
America is also a place that encourages and celebrates individuality (for better or worse). At the same time, it's a country of sub-cultures and mixed cultural identities.
So you get people buying cars that they think fit their personal identity and their sub culture.
Prius(i?) were a statement for the conscious movement, but it's damning that Toyota never pushed the bar further and explored (until recently) other cultural identity vehicles as eco-conscious (like Tacomas, imagine how awesome that would be!).
tl;dr - cars are critical to American culture, and not just as practical tools for moving around. Even if you think you buy a prius because you "don't care" - that unto itself is a cultural and style decision you are making to express yourself. Hoodies forever.
side rant: Why Tesla is the only car company to make an electric (or even hybrid) that doesn't look like vanilla boring practical dogshit is beyond me.