[Someone]

I would guess many Twitter users don’t care much about discoverability of their other work, if they have it in the first place.

If they have it, I think “self-hosted” isn’t a realistic option for the majority of Twitter users. Their other content will likely be distributed between Facebook, Instagram, etc.

Twitter users also can have some long term control over what they wrote: users that want that can regularly download a machine-readable copy of their feed. I doubt many use that kind of backup, though.