so it’s looking like the answer is “because Mastodon is poorly designed”
Thanks @lennon for this link: https://voidfox.com/blog/the_sisyphean_effort_of_activitypub_migration/
It *seems* from the sound of it, there’s effectively no way to transfer keys between instances. Which smells to me of a design problem so fundamental that this is the first time I’ve had thoughts about leaving this place and trying to do without