Chaosmancer
Legend
There's a huge difference between a painting and a campaign setting. A painting is a single, still image. The artist quite probably had important reasons for painting in a particular style, but those reasons are not actually all that useful for painters. For instance, abstract painting came about because the invention of the camera made lifelike paintings pointless. Why spend hours or days or longer on a painting when a camera could do the same thing in no time at all. Instead, painters decided to try to capture movement and emotion and the play of light and color at different times of day, which cameras at the time couldn't do. Knowing this bit of Art History 101 does not make me into a better painter. Actually getting paint on the brush and canvas does.
Yes yes Analogies are not perfect.
A setting is going to have decade's worth of information built into it from potentially dozens of writers (as well as hundreds or thousands of GMs), meaning it's going to be simply too complex to show how it was made
Is it? I don't think it is too complex. Because I think you are trying to look at this as though it is an encyclopedia. Even if you were making a brand-new setting, attempting to include 200 cities, a dozen factions, three different BBEGs and thirty areas of interest is going to be too much.
We are talking about a chapter. Even if it is a full 10% of the book, we are talking only a little over thirty pages. So, clearly we are going to be looking smaller. Maybe two or three regions, with a major city, and a threat or three in each area. And it would be very easy to start with "Make a location, like the Village of Hommlet. Now, you need a nearby threat to hook the adventurers in, like this abandoned temple" yadda yadda.
--assuming they even know why certain aspects were included in the first place. I doubt that everyone who contributed to Greyhawk (or the Realms, or any other setting) wrote down those reasons. And let's face it, a lot of those reasons would be "because it's cool" or "because my players did it this way" (IIRC, this was the reasoning behind the creation of kender), or "because I have issues that really should have been worked out in therapy." (cough, cough, whoever decided the drow were evil dominatrices, cough). None of these are really helpful for worldbuilders.
Actually... because it is cool is a fine reason for a world-builder, spoken as a world-builder. But I'm talking function. You need to create towns and cities, why? Explain the reason. You need to create adventure spots, why? Explain the reason. You need areas with multiple threat levels, why? Explain the reason.
Sure, maybe they don't have Gary Gygax in a glass jar to pick his exact memories for why he did something, but I'd also say that I doubt Greyhawk is so poorly put together that it isn't impossible for a world-builder to look at it, figure out a reason, and then use that to explain to someone the logic behind world-building for the game.
You aren't trying to feed them an encyclopedia of everything that has ever happened in Greyhawk, but you are looking for examples of "how do you build an effective BBEG", "How do you make it interesting to travel from the low-level village to the mid-level city" and things like that.