Evolution makes adaptations to what it already has, unlike a sentient designer who might create an optimal design then look for ways to implement it.

If you could design a human from scratch, it might be most efficient to give northerners a D synthesizing pigment that's opaque in the necessary wavelength, resulting in the situation you describe.

Instead we have protective pigments near the surface, synthesizing structures underneath, and a need to balance D synthesis against UV damage.