Doesn't it kind of already exists for passports? I can travel with my passport to (basically) any country on earth, so there must be some international support for this.

The format of the data on a passport is unified (https://en.wikipedia.org/wiki/Machine-readable_passport) but the problem is to make sure that the one who presents this string of data is actually the one it belongs to.

Good for you, but, for example, Israeli passports are not recognised by many countries as legitimate.

Well the argument not that it it readily available, but that most countries do have an administration of their inhabitants.

So instead of "There is a good identity", GP should have said "There are about 200 good identities around the world, but if your country happens to not have a single unified ID system you're out of luck"

There are 200 good identities in the world with an authentication system closed outside the official entities.

I have never seen any endpoint for the general population that can be used to authenticate a citizen