There are 500 million posts on Twitter every day. Do you read them all? There are 2.8 million subreddits. Have you browsed them all? The internet is big. You’re not going to be able to follow everything everywhere all at once.
That is not really the point though. Suppose there are 2.8 million forums on Kbin or Lemmy. If you join an instance of any ActivityPub app, can you talk to any of those 2.8 million? Can you interact with all of the precise subset of forums that interest you? You can choose an instance to join, but you have to know ahead of time all the instances you want to interact with, and your interests might change over time. Or you have to create and maintain multiple accounts.
This isn't exactly sustainable for individual users.