They have the main thing masses often look for: unity (as in the state of being united, not the company). Like it or not, federation is a brand new concept that regular users can't wrap their heads around.
See my other reply. Regular users don't understand, nor care, nor even think about it. To the mainstream, of everything you've listed, only email is a common concept, and even then you have some major congregation with very few providers.
It doesn't matter that your bubble is used to it, because it represents a risible percentage of the mainstream.
You have a point. I actually don't want mainstream people in here. Because they suck so much as the youtube comments used to show before it implemented upvotes.
I can guarantee you regular users don't ever think of email as a federated service nor even begin to understand how it works behind the opaque client page.