Speaking as someone who used to answer regularly and now doesn't at all, it's because the moderators have gotten insane and taken the fun / interest out of it. So at least in my case, your thesis is incorrect.

> Trying to add jokes to Wikipedia main page content is a similarly fruitless endeavor.

wikipedia is routinely used as a site with a toxic culture that succeeds inspite of, not because of, its community.

Imagine how great Wikipedia would be if it the userbase didn't have a significant proportion of pathological arseholes.

Would it be useful if while reading about a solar eclipse I would find jokes about people pulling their pants down and mooning you?

The content and vision of the founder for that content is what draws people to Wikipedia and stack overflow.

That it takes people who occasionally need to be assholes in order to maintain that vision and ideal is a failing of communication to the out of sync contributors... and those contributors to follow that ideal.

There is nothing stopping anyone from forking the content stack overflow or Wikipedia and providing a different vision. If more people follow that new vision, it wins and the world shifts to a different site (that is what happened with experts exchange).

The main problem with the fork however, is getting the people who contribute good material and don't want to have to read jokes about exposed buttocks on the moon or favorite comic strip amidst documentation for Java.

The problem is that "people who occasionally need to be assholes" overlaps significantly until it is a constant stream of people in asshole mode (who are just having their asshole moment of the day). So it's disingenuous to say that people are "only occasionally" being assholes. There is definitely a unified asshole front, even if it's only being manned by part-timers.

Yes, it would be fun. The problem being that there is no scale to measure humour and taste, and that moon joke doesn't seem good. But I regret that, in the absence of a scale, our culture decides there shouldn't be humour. I'm curious whether it's a USA thing or whether an Asia-originated or Africa-originated Wikipedia and StackOverflow would have resulted in different rules.

I don't know what the grandparent meant by jokes, but the current problem -- the malignant influence of a small number of authoritarian Wikipedia editors -- has nothing to do with cracking down on silliness.