Perhaps it’s because for the target audience it’s basically “anything that looks unfamiliar” (since if it’s familiar it can’t be authentically fantasy/medieval /s), and for the visual designers Eastern/Central Europe is vaguely unfamiliar enough while still being recognizably Europe, re: general flora/fauna, buildings for those who don’t understand architecture, etc.
It’s kind of like the concept of diversity. BBC’s recent Dracula miniseries sucked (ha ha) for many reasons, but the thing that made me low-key boggle was the vampire killer nuns in Budapest (not, in fact, shot in Budapest, the city can apparently stand in for every Central/Eastern European city except for itself 
). They had Asian nuns, black nuns… but no Romani nuns, perhaps because they wouldn’t register as a different ethnicity for US/western viewers. Even though here “diversity” in this context would start with including Romani people.
Yes, I’ve been noticing this in the past years, and I always wondered if it’s because the audio is mixed to home theater systems, not regular TV…
