@julian@flancian@renchap Yes, the primary reason why full migration is not implemented is because it is very difficult to do (the solution I proposed is FEP-ef61; it probably needs at least a couple of years to mature before Mastodon can implement it). However, in some cases interactions don't need to be preserved, and various tools for working with Mastodon archives exist, for example https://posty.1sland.social/
@flancian There are also protocol-related concerns. Setting up copies of existing Mastodon posts on a different server changes their ActivityPub IDs, so past interactions would get lost. So you can't meaningfully "move" posts - that would require them to have decentralized IDs, something that @silverpill does work on.
I'm with you that copying post content should be part of account migrations though. @renchap once mentioned that Mastodon wants that too, it's just not urgently prioritized.