You bring up an interesting point, a subject that could become debatable.  From what I've seen, the ethnicity packages seem to be delegated to third-party vendors who are wise enough to see the need and fill their pocketbooks.  But to drive a point, it is difficult to create those textures in terms of detail and color.  For example, Blacks come in various shades of brown, from very dark to almost white.  Latin American Hispanics also have numerous blends of indigenous natives and European settlers, in addition to African slaves introduced into the colonies.  I believe that the most challenging skin to reproduce would be from Blacks, especially the hand palms, foot soles, and skin specularity.