Think if it this way. Charisma has nothing to do with appearance. Everyone in Thera knows they are evil geniuses, so are you going to trust them? Charisma stats for a race I think takes into account a large part how a race would be perceived by other races in thera. And I know the flaw of humans having 23, but that's just a function of how stats work for them.

Charismatic is not really something a dark elf is, because no matter what people would never trust them or be persuaded by them. Simply because you know they are evil geniuses that will stab you in the back.