They enter the Fediverse, use ActivityPub, begin adding some “extensions” to it and suddenly everyone has to play cat and mouse with the new extensions because Meta has 90% of the users. Then, some time later, Meta says they don’t see the need to be federated anymore and pull the plug, disconnecting everyone in the Fediverse from 90% (or more) of the users and from the communities that live on the Meta side.
Just look at what happened with Jabber and Google Talk.