Because it was utterly inconceivable that there might be a FANTASY world without white people, or a post-apocalypse world without white people.

But what do you mean? IT'S FANTASY! THOSE AREN'T WHITE OR ASIAN! THEY ARE FANTASY PEOPLE. Who look white of course, but we know people, as in without any marker, look white.

(Do you mean people of color would want to be in a world without white people?)