Acting is not about changing the appearance.

Race is relevant since it tells us quite a bit about someone and people of different races are and have historically been treated differently by society. Japanese people, for instance, were(still are) quite xenophobic.

Why not cast an african or a white person as the Emperor of Japan then? Can’t they act?

Let’s have a white Martin Luther King. Let’s make black people play slaveowners and whip other blacks around, surely they can act quite well.