This has nothing to do with player importance of course, often a group's champion could be the top dpsers or the amazing healers or what have you.
But from a "logic" perspective, there has always been heavily armored people being acting as a champion, whether it be a phalanx or medieval knight or even a motorized gun (a tank.) These people or things are way, way too tough to ignore. You can't run past them, they define the line. The only thing which is unrealistic, is that there's only one or two of us on a boss at any given time.
I just view it as we're too dangerous to leave alone, and we're too tough to kill.