White colored surfaces reflect sunlight back more efficiently and keep you cooler. Why does white human skin perform the opposite way?

You have to consider the frequencies of light that are reflected. White reflects a lot of VISIBLE light but not the INVISIBLE ultra-violet light that causes harm to humans in excess.

White skin evolved in humans who moved away from hot, sunny places because it has an advantage in creating essential Vitamin D in the reduced light levels. Dark-skinned people may suffer Vitamin D deficiency if they live too far away from hot, sunny places.

The difference in skin colour is brought about mainly by melanin levels.

Melanin  reduces the damage done to the skin by the sun. It can also decrease vitamin D production.

In regions where there is a lot of sunshine, native populations tend to have more melanin for sun protection.

In regions where there is less direct sunshine, native populations tend to leave less melanin as they need less sun protection and need to maximise sun exposure to get enough vitamin D.

Because we're not white?

Look at your color key, then look at our skin.