Why is it that when you think about some bands you just think of their lead singer?

As far as NIN goes, Trent Reznor IS the band - he creates all the music. Ya, but that's just how it goes. The lead singer is the prominent one because he/she is the one you see the most in the vids and usually are the ones who write the lyrics, so it's fitting for them to sing them, too. You only hear that peron's voice, so you learn to ociate the band with that person, if that makes sense...