Which demonstrates ignorance. They aren’t interchangeable at all. Greyhawk definitely has its own flavor as does FR and your assumption is based on a lack of knowledge (the definition of the word).
While FR is a kitchen sink setting, one could easily think it’s a fantasy Europe, based on looks alone, it’s more Canada and that’s very hard to put a finger on why. It doesn’t really have a France or Germany or Norway etc. you could make an argument for an England with Cormyr and Moonshae is definitely an Ireland and Cormyr has those Arthur Celtic influences they’re so far apart that they have distinctive cultures that fall apart but the whole thing is blasted apart as you look at the broad expanse of the continent of the main part of the Forgotten Realms. In its fringes to the south and south east you get into the more Earth like cultures but the rest, particularly the original OGB parts of the Realms, are what the concept of generic fantasy would grow from, not because it was generic but because FR would dominate fantasy fiction after its release. It’s not even Tolkienesque. That is Dragonlance.
Greyhawk on the other hand is very different. It’s as generic as the DM wants to make it or your imagination makes it. It has some very ripe and imaginative cultures but very humanocentric so the elf, dwarf, etc cultures are barely surface scratched. The setting is very sword & sorcery but with chivalric and heraldic traditions proudly on display. The Great Kingdom is very much a Holy Roman Empire divorced from the Church kind of vibe because that is over in Veluna I think. The city of Greyhawk is very Lankhmar and in general it’s a very Wild West, Conan the Barbarian, Fafrhd and the Grey Mouser kind of setting. They would feel very at home there. It’s not Tolkienesque. It could be argued that it is Eastern European and bits of France and Italy meet Fritz Lieber. Basically… The Witcher if you accelerate GH to the Wars era.