That's because it's not.
When most people discuss "DoD,"* they are talking about the classic I3, I4, I5.
Pharoah (later I3) was first created by Hickman in the 70s, and then expanded to the I3-I5.
It is setting-agnostic, although, if it had to have a setting, it would be Greyhawk, because all modules from that era were either explicitly Greyhawk (like Saltmarsh, Tsojcanth) or had to be considered a part of the setting.
After the ouster of Gygax, TSR first published FR in 1987- AFTER the publication of the individual I3-I5 modules (five years after).
At the same time, they put out a compendium book (I3-I5 combined into a single volume called "Desert of Desolation") that did the following:
A. Slightly expanded the material, and made it worse.
B. Shoehorned the setting into the FR.
This was the first, and earliest, example of how everything good and fun and happy eventually gets pushed into the FR and becomes sucky.
...IMO. Anyway, don't use the 1987 material! Use the good stuff. And don't forget- you can easily set the generic 1983 stuff in Mystara, or whatever homebrew you want ....
At least, until the FR gobbles that up, too. There has to be a blank space on the map of Faerun, somewhere, and if not, I'm sure we can just Plague Sunder one, right?
*To be clear, and to put aside the jokey aspect for a second, the confusion naturally arises because IMO the majority of people use DoD as a way to refer to classic modules, which explicitly are a trilogy that refer to the Desert of Desolation ... in fact, even the covers of the module would say. "The second module in the DESERT OF DESOLATION Series"; however, other people can, in good faith, refer to DoD as the first compendium that was called that. There isn't a wrong way to look at it.
The FR version is the more interesting, because it's ties into a broader, more interesting setting, you can use old Empire, expand the adventure into the Celestial Nadir (a massive demiplane where Imaskar stored all kinds of stuff, from treasure, monsters, and more), there is so much potential, that is why if they do DoD for 5e, I want them to do their own expanding of it,something like the Sandbox playof SKT,but for the Old Empires region.