Because the DM's role is different, right? They effectively are the judge, jury and (figurative) executioner. Even in a creative endeavor like a sketch comedy show (let's say SNL), there's still going to be someone like Lorne Michaels who oversees things. Otherwise, it's chaos.
I'm not saying a group of people can't all come to consensus on their own sometimes, but I am saying that they won't come to consensus all the time without a DM.
I don't know how many times at work we were told there was no manager, no project leader. But before too long there always was a de facto leader, usually the one that spoke loudest or refused to compromise.