It's not federated by any means, but if you want to replace FANDOM wikis with other equivalents, Indie Wiki Buddy is a great extension to have on hand.
There's options to remove FANDOM from search results in favour of other options, and they also allow you to redirect to the Breezewiki frontend for FANDOM to get rid of all those shitty ads and UI, which is legal considering the contents of FANDOM pages are still under the Commons.
Cost would be a big factor ... Fandom got big by being free and eventually replaced (or heavily customized) mediawiki to the point it's unrecognizable.
The ability to self-host your own FOSS wiki already exists and has for over two decades. It's called MediaWiki.
You could have federated accounts I guess but do editors on the Doctor Who wiki really need the ability to see posts on Mastadon or edit pages on the That 70's Wiki?
Discovery. The current state of google dooms such small wikis. They will have zero traffic. Google has been overtaken by AI slop, so if we want to be relevant, we have no choice but to federate
In addition to discoverability, I'd say it provides a happy medium between letting every rando with an IP address edit a page and requiring account creation. Part of the point of the fediverse is to have (almost) everything in one place under a single account while still keeping things decentralized.
Can you elaborate on "discoverability"? Finding individual subject wikis has never been a particular problem for me. Even ones that don't use Fandom, provided they are at least active. Just googling "<insert subject> wikia" (I know. I can't let it go) always gets me what I need.
Can't say I see an advantage to universal accounts (I see more disadvantages), but if that's the big selling point and people really want it. I'm not opposed to having it, i've just always treated it as a mild novelty I never use.
As for decentralization, it has already been solved by MediaWiki. Which is GPL and (can be) self-hosted.
Easy hosting isn't quite the issue. Dokuwiki is trivial to self host. What I'd like something that's a happy medium between requiring account creation to edit pages and letting literally every rando with an IP address go to town.