Actually the observer and explorer links redirect to external sites (fediverse.observer and lemmyverse.net respectively)

It would be odd to single out just the biggest server instead of letting people decide themselves; I have a feeling this exclusion is to do with .world’s users having a beef with the folks running .ml 😅 (and vice verse - join-lemmy.org seems like the official Lemmy site, so ran by the .ml ppl)