@res260 because of a cultural Fediverse thing. People started off trying to make servers that simulated existing commercial services: Fediverse Twitter, Fediverse Instagram, Fediverse YouTube. So they made servers that only handled one kind of data or activity. On top of this, Mastodon already had a "better" API when it implemented ActivityPub, so it stuck with it and never implemented the ActivityPub API. This also spread the incorrect idea that the ActivityPub API wasn't good.