They can't. It would destroy their moat and since they are already unprofitable it would be the end of the company.

Twitter, facebook, youtube treat their users as capital. their value is that the users can't move to simething else. If they would implement activitypub they would effectivly burn money

Honestly that's stupid. Sooner or late people lean towards choice and freedom and when that happens monopoly breaks.

I can't think of a single way that would benefit them, so if you don't mind I'm going to flip the question around.

Why should Twitter implement activitypub?

Saw a post somewhere that alluded to a company doing what I would call the Microsoft Way™ (and other companies) type of thing where they Embrace, Extend, Extinguish on some burgeoning competitor or space they want to get into.

Embrace: the open standards, maybe offering mastodon hosting for a small fee using twitter cloud services, offer experts to help move the standards forward.

Extend them selfishly optimizing your own thing like maybe a federated twitter/mastodon platform that bridges between the two, providing messaging across, ad-network sharing, etc. Keep adding to the ActivityPub standards with things people want and hosting becomes easier to choose Twitter hosted Mastodon.

Extinguish: after some time spent accumulating users, stabilizing adverts, establishing an ad-network, moving standards forward, etc. start charging more for mastodon hosting or allow users to migrate easily to twitter and shut the federated system down.

Ah, that magical word, “just”…