Fantasy of manners is heavily -- maybe exclusively -- drawing on nineteenth century societies, even if they get a lot added in the process of becoming an imaginary land. Part of this is the influence of Jane Austen, who managed more than any other writer to write classic novels with, at most, off-stage elopements, and duels we do not hear of until they came off without consequence. Fantasy of manners works tend to use more dramatic elements -- wisely, as Jane Austen is a very rare genius -- but the manners side often derives from her.
And then the nineteenth century was a very convenient century. Aristocrats and kings still held a lot of power, but social mobility had increased, the industrial revolution was starting, etc. -- and so people were immensely class-conscious. You will search in vain for something like Chaucer's Canterbury Tales, where people from immensely different social strata all went off together. And when wealth is floating about, manners are a good way to enforce social rigidity, and there is much need for enforcement. (Then again, once they have mastered the manners, they are a positive aid to social climbers. An earlier example: in medieval Latin, "knight" is rendered "miles". Once upon a time, this was because all warriors were knights, and knights were merely warriors. The revival of commerce had given rich merchants clouts, and the knights had invented knighting to set themselves apart. What it meant in the long run was that anyone who could get the ceremony could be a knight without being able to lift a sword.)
Plus, of course, there is the little matter of world-building without it. Complicated systems of etiquette are even harder to devise than most systems that world building needs. Much easier to rip off the real world, even if you have to scrap off the serial numbers.
Though China and Japan can be good sources, too.
part of bittercon .