There's a few things going on here in terms of practicality.The whole idea that a campaign setting needs to have real world cultural analogs is lazy. I excuse Mystara for doing this because it sort of did it first before it was a tired trope, but I would be 10000% open to a hypothetical resurrection of Mystara moving these cultures away from their real world influences toward more original, fantastical presentations.
Real world cultural analogs are basically easier for most people to use and get a handle on. There's less to remember. Numenera has "The Steading" for it's far future setting, and for some reason the Steading has about 10 different nations. Damned if I know what the major cultural differences are or how to get a handle on them. Now in a fantasy game if one was fantasy France, fantasy Italy etc then it would be a whole lot easier.
Real world analogs basically allow our real world knowledge to do a lot of the work for us. Sauridyya is fantasy Arabia. I need to make up some stuff quickly for a game session - so I fill it in with oasis, cities with bazaars, bedouin tribes, a magical cave, camels etc. It's much easier in terms of content creation both for developers and for individual GMs who have to make something up when the players decide to go to the blank part of the map.
Of course, the tension here is that it's easier because we draw on real world knowledge, but the problem is that our real world knowledge might be full of cultural stereotypes which then manifest in the game*. Even if the developers have done their research on Arabian history, have individual GMs? And what about their players?
*And of course there is also the problem that if you remove these things, you might lose some of your appeal, because D&D fans (even the progressive ones) are primarily fans of pop culture and pulp and not of say, serious history.. If I'm making a game set in mythic 9th century China, players will probably want to have magical Kung Fu powers and will complain if the game does not in fact, include such. So rpg design has a real minefield to navigate (and this is one reason I don't have much faith in sensitivity readers.)