Big Social are no longer the innovators they were.
In actuality, for the past 10 years, most innovations in social media have been created by small, nimble players—which Big Social then attempts to either acquire or copy.
In essence, they've replaced R&D with M&A.
Thus, they probably have nothing new to add to the Fediverse.
The Fediverse is one thing that Big Social cannot acquire or copy.
No one owns the Fediverse, so that's not an option.
And to copy or integrate the Fediverse is to kill their own cash cows.
In essence, the Fediverse targets the chunk in their armour.