I'm going to use the kryptonians as well as the saiyans in this explanation because bluntly put: they are both (superpowered) aliens that look like humans.
In a real world scenario, if we are to believe that kryptonians and saiyans would look nothing like any race of the humans on earth, then there's absolutely no way to fathom what their facial structure would look like other than knowing that they'd have two eyes, a nose made of cartilage, two ears, lips, and hair on their head. With this being said, here is my logic. When Superman was discovered by his foster parents in Kansas, that race of people (white Americans) were the first people to feast their eyes on this humanoid alien infant. Because of that, they are the first humans to interpret the Kryptonians' likeness to the humans and thus, that is why it is appropriate to cast a caucasian as Superman.
Goku on the other hand, was discovered by an elderly Asian man in his universe. It's quite clear that DBU's earth is different than ours, but it's also quite clear that there is a western world and an eastern world in it as well, made up of black, white, Asian, brown, and Native American peoples in made up versions of the real earth's countries. With that being said, elder Gohan was an Asian man and since he is the first human to be exposed to the alien humanoid race that is the saiyan, he is the first to interpret their likeness to humans, and thus should be seen as an Asian man.
all this said and I've said this in the film franchise thread, I've found the perfect (in my opinion) person to play our Goku: Fizo Omar












and yes middle east&India is a part of it.