Long complicated reason: mastodon currently uses a legacy attachment manager package for Rails and it's ridiculously complex to upgrade to ActiveStorage or something else without needing to reupload all media (which would potentially be prohibitively expensive for server operators & require publishing update to all existing notes & actors containing media)
- https://github.com/mastodon/mastodon/issues/5729
- https://github.com/mastodon/mastodon/issues/7414
- https://github.com/mastodon/mastodon/issues/30352