Because people want to feel "cool." As corny as that sounds, it's true. They want the "respect", even though seldom do anyone get respect by banging their head to music.

It's funny though. One time in my car the car next to me had these huge subwoofers. SHE, not he, played her music so loud my car was literally rocking. I tried to drink some hot chocolate, but it fell on my lap.