It’s stupid that it’s not there, though, and I suspect it will get fixed at some point. Basically, some concerns were raised about people using it to doctor timelines (but they can already do that by setting up a single-person instance and messing with the database), and the new instance can validate the signatures of the posts anyway, so they’re no less secure than posts from other accounts received via federation. If anyone is really concerned they could slap an “imported from …” banner on old/migrated posts so people are aware they were not originally posted there.

Sadly, while you can export everything, you can’t import posts in Mastodon without having direct access to the database and poking about.

You can import following lists, bookmarks, blocks and mutes, though.