Key word is "ethnically and culturally homogeneous enough". And since we're talking about political policy here, domestic image is what matters here. The actua

Key word is "ethnically and culturally homogeneous enough". And since we're talking about political policy here, domestic image is what matters here. The actual numbers are not known because they aren't collected as part of the census, but the government estimates that upwards of 98% of the population is "ethnically" Japanese. Furthermore, many of the ethnic Koreans that you mention, as well as ethnic Chinese (the next biggest group, as I'm sure you already know), are culturally fairly assimilated, even if many are second class (non)citizens in the eyes of laws and much of society.