Because people have yet to overcome the impossibility to accept the "optional" concept of content that is there NOT FOR EVERYONE. They think that if it's there they are forced to play that way. They fail to understand that they're not the only people in the world and that maybe the majority would use such a feature . It's a feature that it's on top of the base experience, entirely optional, and not necessary to achieve end game.
Blizz said the Open world feature is “optional”? Where did they say this? I was under the impression you didn’t have a choice in sharing whatever world you’re in.
It's optional in the sense that there is very little in the game that is built around group play; at the time of the demo, the only thing that needed more than 1 player was the world boss. And as stated in the other thread, it is not a pervasive element.