Why do most of the core gods have nationalities listed?

This always bugged me. Mainly because it seems to human (or at least, humanoid) centric. As far as I know any race can theoretically worship any of the core gods (correct me if I'm wrong). Also, presumably most of the core gods (with some exceptions like the four Ascended) existed before Golarion even formed so assigning them nationalities makes even less sense.

IIRC, those locations are "Centers of Worship" which is to say "places where there is a notable concentration of followers of that deity, including possibly large temples and official organizations".

An individual can worship any god anywhere, but if you worship Abadar and live in Galt you probably want to keep that quiet and even though there are a bunch of Desnans in Nidal, it's not like they can put up structures for worship or congregate.

Ok that actually makes sense.

I was under the impression that these cultures were where the faiths of these gods got started in the Inner Sea region: the first people to worship Abadar were Taldans, while the first Pharasmins were Garundi, etc., and their worship spread from those peoples as they founded nations and interacted with others cultures. While, yes, deities like Abadar and Shelyn were worshiped by the ancient Azlanti prior to Earthfall, the books don't really concern themselves with stuff prior to Earthfall for the average PC because to them it's ancient history.

Inner Sea Gods, which I believe is the definitive book on the core 20, lists two separate fields. One is "Centers of Worship" which is "regions around the Inner Sea where worship of that god is particularly common" and also "Nationality" which is "the nationality, race, or ethnicity with which the god is most commonly associated."

So like even though Desna is definitely not Varisian (she's ancient and alien and probably an Outer God originally), people expect Varisians to be Desnans, and if they were looking for a Desnan cleric they'd probably go looking in Varisia, even though "being the goddess of travel" her people get all over (e.g. Kyonin, Lands of the Linnorm Kings, Nidal, Numeria, River Kingdoms, Ustalav, Varisia).

It's an idea that we thought would help contextualize the deities in the early days, but I suspect we'll be abandoning it going forward for other methods of doing so that aren't so weird.

Oh, I see! Thank you for the clarification, James! :)