If there was some kind of organization or working group that could adopt extensions into a common namespace, then sure, you might have a point, but right now the development of extensions is led by individual projects and implementations. There was LitePub for a while, but that was more about dropping JSON-LD and adopting HTTP Signatures. Perhaps the FEP process could manage its own JSON-LD context file, which could make it easy for projects to adopt all the FEPs at once by simply including one additional context. But this isn’t strictly necessary. It would also save having to add toot, pixelfed, sm, misskey, and so on, though… and it could be a more neutral/agnostic way to handle extensions that are developed by multiple projects in collaboration, so that no one project claims the definition unilaterally. But this isn’t really important or significant at the end of the day.