Says who? White skin is the result of a reduction of the pigment eumelanin inside the skin cells. Albinos also have white skin, but they totally lack the ability to manufacture eumelanin, usually because there are mutations in the genes that are responsible for manufacturing eumelanin. For example, the OCA2 gene codes for an enzyme needed to produce eumelanin. If there is a mutation that disables this gene then a person will have light skin, without any autoimmune disease.
Many people with light skin tone still has the ability to produce eumelanin, especially in response to exposure to the sun Their skin turn darker when it is exposed to the sun. Eumelnin protects the skin from cancer causing UV light, in regions of the world where people did not wear clothes because the climate is hot. Light skin evolved when dark skinned Africans migrated to cool climates and they began wearing clothes, about 40,000 years ago. The clothes protects the skin from skin cancer, but it also blocks out the beneficial UV light that helps the skin manufacture vitamin D, which is needed to prevent rickets.Rickets can cause malformed bones, broken bones, even death. Therefore light skin became an advantage in these regions, even though light skin remains deleterious in Africa. Africans with albinism typically develop skin cancer by their early 20s. That is why light skin is so rare in Africa. It is nearly universal in cold climates because rickets kills more people there than skin cancer.