There's something funny about that sentence. "Japan has a monopoly on cursed energy". While there are characters like miguel and toji's contact who iirc was korean 🤔 so while Japan's cursed energy is the strongest and what was the sentence? "People of this country are...?" It's not like the the rest of the world doesn't know what ce is or doesn't have It's own history on it. I honestly think gege chose this way to make it easier for him to write. I don't blame considering how much brain power he has to use for the power system and how tiresome researching can be on a deadline.
¯\_(ツ)_/¯ it’s manga, y’know? Gege makes the rules, and Gege said that most sorcerers come from Japan. American comics—hell, most of the media I consume—depict US-centric stories that downplay the international elements at play.
It makes sense to write a setting you’re familiar with, especially given that JJK’s audience was all Japanese before it rose to popularity to be come an anime.