For the same reason that, as others have noted, the ethnicity of the actor doesn't match the ethnicity of the character on a regular basis no matter what the ethnicities in question are. Usually it makes no difference, like in your lame Galaxy Quest example. The character has no discernible specific ethnicity at all, unless you grab onto the surname and scream 'yellowface' about it there is no clue that anything out of the ordinary is occurring there because nothing out of the ordinary is occurring there. Actors act. Unless there is something absolutely demanding of a specific ethnicity in the role the best available actor is going to be cast. The obvious example of roles calling for specific ethnicity in the actors would be if you are casting a show with plot elements like 'problem faced by biracial couple' you would pretty much have to fill the roles of the biracial couple appropriately. This 'oh the character is named Lee, that demands an Asian actor or it is grotesque yellow face' theory is utter garbage.