Global migration outpaced evolutionary adaption a long time ago. Many peoples have adapted to local UV conditions, but can now jump on an airplane and are instantly in a completely different environment.

Additionally, if you've ever seen a portrait of a human in the UV spectrum, you'll notice how shiny they look. I'm sure modern people have much less protective oils in their skin as a result of increased bathing required by societal and sanitation norms of modern urbanized habitation.