Joachim Pense is right that it’s neither emergent nor inherent, but selected for by moderation.
However, the extent of humourlessness really varies by topics. Topics inherently select for different balances of the factual and the anecdotal. Survey Question is awash with anecdote, and humour follows it naturally. Where the topic inherently allows for it, a community of practice can develop around prominent answerers, so that there can be emergent humorousness. But I’d argue it’s quarantined.
Some of those polymaths also try to inject levity into answers, so long as they are still providing relevant answers. I think a fair few of them succeed.
See also: Why is there very little humor on Quora?