And then, again, where are the monocultures, even in 1e ? Yes, the PH is fairly generic but so is the one in 5e, but let's take for example Greyhawk, as I've pointed out, it's the same as Eberron, there are multiple and very varied elven cultures, for example, from Celene to Ulek including some very reclusive cultures like the Grugach and the Valley Elves, and at the same time you find elves in large numbers in a lot of countries around the map, for example in Geoff, Keoland, etc.
As for their creation, a lot of the demi-human races were created as derivative of Tolkien, simplified for use in the game, that's all. It was not even a question of culture...
I agree re: Greyhawk. It did indeed immediately start to move away from monocultures, at least with certain races.
Part of the issue is the constant return to monocultures every edition, most spectacularly to my eye with 3E. It's approach that to me, has never made sense. It's not something every RPG does, and I think in 1st it maybe made sense as a first step, but every edition after, starting with monocultural depictions has made less and less sense.
2E had the most branching out from monocultures (imho), and it had from the very start (c.f. Taladas), so it was particularly shocking to see 3E "reel back in" the races (and classes, but that's another discussion), and then it never really reached the same levels of varied cultures for races, and whilst 3.XE added tons and tons of races, virtually all were presented as fairly narrow monocultures. Even ones that it made no sense for.
I think going forward it makes sense to not make the same mistake yet again, and return to monocultures hardcore. Whether than entails detailed faction rules or something else is open to debate, but I don't really want to see the same sort of "Elves be like this, Dwarves be like that!" cultural stereotyping this time. It doesn't really have much to do with real-world racism re: the core races, for me, note (that only comes in as you spread out to humanoids), it has more to do with starting from a place that makes sense to work from, rather than reverting to something that you then have to spend half the edition (or more) moving away from.
Re: Tolkien-derivative races, indeed, but again, we saw people take them pretty far from that pretty early on, so it was curious that they kept reverting (except Halflings/Gnomes, who were in flux through 3E-5E to a greater or lesser extent - still monocultural or the like for the most part, but what that culture was varied a bit more).
To me, factions are very world building dependent.
And the detailed races of previous and current editions aren't? I think you can genericise factions pretty easily, and it might actually be nice to have some be setting-specific and advice for creating your own for your own setting and so on.
D&D doesn't really have that core shared campaign setting as standard.
Yet 3E, 4E, and 5E have all acted as if it does to a large extent. I mean, how you feel about that is up to you, but it's certainly not novel.