Threads (for better or worse) demonstrates that that’s not a fundamental obstacle for fediverse microblogging.
If someone wanted to launch a Mastodon fork with algorithm-driven content discovery, they could do. Just as with Lemmy/kbin/mbin, the beauty of the fediverse is that different servers can take quite different approaches to use experience design whilst still maintaining compatibility with the rest of the community.
“Never” is a strong word. API translation is a technical hurdle, but rarely an insurmountable one. If Blue Sky wanted to add an ActivityPub interface to their platform, they probably could.
This issue isn’t technical per se; it’s a matter of priorities. Blue Sky doesn’t want to federate with Mastodon/Threads, because they want users to switch to their platform.