Closed networks are in a heated zero-sum competition for users, so your reach is limited to people on the same platform.

Email, the web’s original open protocol, is used by more people than any platform or social network that has been invented before or since; because it shares users rather than competing for them.

The ActivityPub network works the same way: You get access to an audience of every person across any platform. Open networks grow larger because they don’t depend on the success of any one company.