Probably because helpful people will be driven away when they open a thread and see contrarian attitudes. But contrarians that open a thread and see helpful attitudes will just be contrarian towards both OP and the helpful people.
So yes, there may be effects in play like zero-sum thinking, anonymity, ego, obstinacy, or self-selection for strong opinions or real-life jerks.
But it almost doesn't matter whether contrarian attitudes really are "very common". Absent a force that mitigates this unbalanced outcome matrix, it's almost an asymptotic statistical certainty that any Internet thread with enough participants will have enough contrarians in it that the entire thread (and the dominant strategy for anyone who wants to participate in the thread) devolves towards contrarianism.
But it almost doesn't matter whether contrarian attitudes really are "very common". Absent a force that mitigates this unbalanced outcome matrix, it's almost an asymptotic statistical certainty that any Internet thread with enough participants will have enough contrarians in it that the entire thread (and the dominant strategy for anyone who wants to participate in the thread) devolves towards contrarianism.