The reason is simple. In the PuG environment, everyone assumes everyone else has already beaten the fight and can carry each other through the fight. So the minute something goes wrong it goes from "ez carry' to "crap now I have to try hard to make this work, well if I wanted to do that I'd join a static and put 40 hours a week into raiding! Why can't these idiots carry me? They've already done the fight right?"

Also literally anyone can say "i've researched the fight, I know the fight" when they have not. Years of abuse by lazy, sorry players has lead to intolerance. Slack jawed, glassy eyed maggots that want to ride on your coat-tails to loot and glory have been plaguing the PuG scene and the tolerance for wipes and failures and newbs has long since been erased.