Wikipedia is somewhat federated, just not in the same way that the fediverse is.

Each language edition of Wikipedia is its own little island that connects with the others. Then there's Wiktionary, Commons, Wikisource, Wikibooks, Wikiquote, Wikisource, Wikivoyage and Wikidata. They all have their own policy, moderation, culture etc. but they all connect with each other.

They all link a bit more like how IRC servers come together to form a network - it's a closed system of federation rather than it being open to the world - and the Wikimedia Foundation does sit on top providing a level of governance and technical support.