And in fact, Mastodon has a mechanism to migrate your account between servers. It has serious limitations (your posts don't follow you, followers do) but it's the best they can do within the AP spec.
Those limitations are due to the Mastodon software, not the ActivityPub protocol.
ActivityPub provides a mechanism that can technically be used for both post and follower migration – not that I'd suggest using it for that – but Mastodon's account migration feature is something separate. There's no technical reason it couldn't support post migration, except that nobody's put the work (largely social) in to get it past Gargron and implemented.
ActivityPub provides a mechanism that can technically be used for both post and follower migration – not that I'd suggest using it for that – but Mastodon's account migration feature is something separate. There's no technical reason it couldn't support post migration, except that nobody's put the work (largely social) in to get it past Gargron and implemented.