It seems like the big boys have some interest in this relatively virgin field. Makes me wonder why

The pessimistic take is that it's part of Embrace, Extend and Extinguish. As well as the examples there, look up how XMPP ran into problems.

The more optimistic take is that they feel that they can add sufficient features and convenience that they'll be able to poach Fediverse user's back into their now partly walled gardens. I like to think it will go the other way and the most active users will realise that giving all their stuff to a faceless megacorporation for free is a bad deal and they move over here. Easy for Twitter and Instagram users, not so easy for those on platforms that give you a cut of the revenue you generate for your overlords. Hopefully, alternate sources of revenue would make up for it but it likely won't beyond the big stars.