That which is adequately explained by malice. Of course it's not backwards!
PUGs are stupid, right? The hunters roll on expertise rings, the DK needed a +block item, and the ret paladin needed on the SP gear. Maybe they are just incredibly ignorant. But at least as often, the problem isn't that they don't know, it's that they don't care. They don't care about you or anyone else.
So next time you think "wow that person is stupid" consider for a moment that maybe they are perfectly intelligent and they use that intelligence to be an asshole.
Maybe they are also ignorant. You try to explain, "you don't need X stat." They ignore you. It's not because they are stupid, it's because they don't give a shit about you. Why should they care what you say or if you are right? They want something and they will take it.
What brought this on? Too many times hearing that this or that famous person is stupid. They probably are not. Name a famous person and the odds are in favor of them being intelligent. Instead they have different priorities, and those priorities do not include you as an important factor.