I don't disagree, but a lot of websites these days choose to offload their login systems to Google and Facebook to certify that a unique human is on the other end. It solves real problems for them like password management, bot detection, astroturfing, sockpuppetry, stolen accounts, and so on.
For these purposes, an actual Real Verified Name is overkill, and a unique identity number would be enough. But Facebook offers that service for Free(TM) so there's not much incentive to the site operator to aim for something more minimal. And with GPT-3 and other systems inching closer to passing the Turing test, this only makes alternatives more challenging for them.
Those are problems in the Wild West that would need solutions if we were to reclaim the net from Facebook and Google.
What links those accounts is not special, it's OAuth2. I don't think there's anything stopping different groups from running open OAuth2 endpoints that just provide a PGP based identity service. I actually wondered why Keybase didn't start an OAuth2 endpoint, because it'd put a big dent in some of the services you described.
I see GitHub OAuth logins in some programmer-centric websites. I prefer that over Google login (and god forbid Facebook). It's just as bad from a login-centralization perspective, but GitHub allows you to control your identity freely, instead of mandating (and more-or-less enforcing) your legal name like Facebook.
(Google no longer rejects or bans you for fake-sounding names, ever since the Google+ controversy, but it still enforces a Western-style first/last name instead of other cultures' name formats or single-part usernames.)
For these purposes, an actual Real Verified Name is overkill, and a unique identity number would be enough. But Facebook offers that service for Free(TM) so there's not much incentive to the site operator to aim for something more minimal. And with GPT-3 and other systems inching closer to passing the Turing test, this only makes alternatives more challenging for them.
Those are problems in the Wild West that would need solutions if we were to reclaim the net from Facebook and Google.