>>96065695
Because despite having vague weeb-fantasy cards, those cards were modeled off of Anime (brand) characters' decks. The larger IP came first for along time - that makes it brand.
Now that the anime has ended, Yugioh is starting to pivot towards being somewhere between brand and vague fantasy. Many more of the themes and cards are entirely game-original and have nothing to do with Duelists or Pharaohs, either implicitly or explicitly. In fact, the flow seems to be reversing with the release of "The Chronicles".