The reasoning behind this is simple. Monkey see monkey must do. Some people are simply incapable of ignoring an "optional" component of a game. Many if not most single player games simply feel less rewarding if you don't do everything in them, if you are not, in fact, a completionist, and the attitudes which that sort of design instill in players can, and does, carry over into the MMO environment.
#1 You're apparently blindly assuming, or presuming, or just over, over-generalizing, that all players with these complaints are former World of Warcraft players without the possibility that some, perhaps even many, of them have never even touched the game.
#2 Telling people who want to be top of the raiding game in Rift to "gb2WoW" doesn't work, and it won't work until Blizzard ports Hammerknell to Vashj'ir along with the rest of the raids. /shrugs.
[humor]As an addition to #1, if you're going to be rampanty intolerant of others' play styles and/or methods of thinking, or simulating thinking, perhaps you should go back to playing WoW. [/humor]