Sounds like you've had too many experiences of that description (I don't hear it that much down here in a much less urban area, but it still happens)! The answer to your first (rhetorical) question is simple--the driver causes his stereo to be like that. The second is harder to answer of course, but if you look at the success of the Beats headphones you might think young people like to feel the bass in their music (think of the scene in Ruthless People where Judge Reinhold is selling the kid on the Dominator speakers), and like their music played loud I remember hearing my first high-end speakers and wondering where the bass was, when in fact I was hearing a lack of mid-bass bloat for the first time. A lot of pop music seems to be geared to sound better with bloated bass as well, but maybe that's just me?
Annoying sounds from cars at stop light
What causes the stereo in the car next to you at the stop light so loud and annoying with over the top bass and how can the driver stand it?