And you think racism has nothing to do with that being the case? Meaning why do you think a country as successful as Japan is not filled with a diversity of immigrants from other races?
I think it's no secret that Japan has a reputation for racism, and that you have it harder as an immigrant.
Still, the board of Mitsubishi is perfectly reflective of the Japanese population (ignoring the gender ratio, which isn't great but in line with the global average for boards of directors)
It's a self fulfilling prophecy, you apply to Mitsubishi or any other company as an immigrant and don't get picked and you tell your friends how hard it is so they don't apply either, and you leave the country instead of starting a family there.