Why does it seem that nobody understands the what makes a genre of music?

Because people tend to find a genre of music that they adore and stick to it. Now, alot of bands change their style of music over the years (Limp Bizkit, Deftones, Korn, Bright Eyes) but they change their image as well. And also, record companies tend to control that kind of stuff so you didn't have things like Bloc Party sounding like andwecamewithbrokenteeth. Most people are drawn to an artist either by the song or the name; the name is in place to evoke what kind of music you're about to hear. That also relates back to the image thing.