Ah this is somewhat incorrect and is in fact the reverse of whats looks to be true...
First ...All people of any color have some "white" skin...
Look at the palm of the hand and soles of the feet of the darkest person
The skin there is "white"...
Is there any befits to that area being "white" on all people of any color?
Or is "white" just the default
The palm of the hand and soles of the feet are areas that do not need protection from the sun...so no need for the body to wast the effort doing the unnecessary of darkening the skin
Conversely light skin people do darker when needed
Coloring the skin look to be an extra cost option not order when not needed, white skin looks to be the lower cost economy model