shrugs Do note that Earth canonically exists in same universe as Golarion and humans canonically exist on multiple different planet without shared ancestry.
Aka while some individual points have specific lore tidbits(Gods of Ancient Osirion took sabbatical for earth for while before they started making come back on Golarion), lot of it is "Probably same reason why there are multiple fantasy planets in universe with same races, gods using same templates"
The real answer is more a question of RPG tradition. Pathfinder and Golarion follow directly in the footsteps of D&D (via DD3.5), a gaming tradition going back to the early 70s.
D&D-esque RPGs have long borrowed from Earth religions and mythology. Already, in '76, we had Gods, Demigods & Heroes from TSR, giving game statistics for gods and other deific beings from a wide range of earth mythologies, as well as a few from fantasy literature.
Some RPGs were specifically set in a fantasy version of Medieval Europe, like Chivalry and Sorcery, and used real-world religions. But the overall tradition was to use made up religions and gods, only occasionally borrowing directly from real-world mythology, like the Osirion pantheon of Egyptian-inspired gods. MaskedGuy's answer is the in-game link, but IMHO it goes further than that.
Paizo wanted a vanilla setting, shooting for stereotypical Earth-like cultures to cover all possible storylines that their general targetted audience needs or can best work with, as well as content ready for APs they want to develop - rather than coming out with a 100 settings each specifically designed for a limited set of interacting cultures (like what killed the original TSR) - they threw it all into the kitchen sink, obviously. Since I created the original hand-drawn map for the City of Kasai, for The Empty Throne module of the Jade Regent AP, and wrote the City of Kasai Gazetteer, I got contributing author credits for it. So I've created "some" Golarion canon.
That said, I publish more limited settings with limited sets of cultures for both Pathfinder and Starfinder now, I don't use vanilla kitchen sink settings for my games, but on a commission I do whatever the contracting publishing is asking...
It is what makes Lost Omens perfect trpg setting for me. Its not most internally consistent world building wise, but its really well designed from the point of view of "you can have pretty much every fantasy genre on same world" point.
Stuff like Numenera is on my favorites setting list, but Numenera is also hyper specialized into specific genre and types of stories.