> Every time I have tried to edit anything in Wikipedia I have run into a very ego-driven community of long-term editors who don't much care for outsiders playing in their sandbox.

This is likely similar to the force that keeps StackOverflow from growing and staying useful. StackOverflow is basically "Answers to programming questions as if it were still 2009" because any new content gets quickly removed as "duplicate" of content that existed a decade ago.

I've seen the same thing happen on OpenStreetMap. You make an edit, and some old-timer comes in and just reverts it. I even once got an angry message there where someone "claimed" an area in a city for himself.

I'm not a Redditor, but I understand Reddit has similar problems with mods.

Not sure the solution. It's hard to develop a community of passionate volunteers, but every time a platform does build this community up, the content calcifies under an inflexible cabal of power-mad volunteers.