Hussar
Legend
I'm not sure I understand your point. Are you saying that you can't paint all of East Asia with a single brush, but you can do that with Europe?
No. Quite the opposite. In D&D we have all sorts of European cultures make appearances. None of the classes are really tied to any specific culture - I suppose druids maybe, but, even that's a stretch. The equipment list covers pretty much all of Europe. The Monster Manual pulls monsters from all over Europe. In other words, pretty much all of Europe is represented in the core D&D books. And, frankly, even beyond Europe as well.
But, the OA books present an East Asia that is a single, Japanese monoculture. The classes are all pulled from Japanese ideas, right down to using Japanese names, the equipment is Japanese, the monsters are almost all pulled from Japanese folklore.
That's the problem.