Why do people say race is social construct when it obviously isnt?

anthropologists say race is a social construct and that small mutations in melanin pigmentation are the only biological differences between the races. I understand the desire to believe this but it just isn't true. If it were true how can scientists identify the race of a skeleton. Why are IQ averages different among the races? Not believing in races is simply ignoring biology and evolution. Humans lived in vastly different environments in our evolutionary history why wouldn't genetic traits be different depending on ones ancestry. We've domesticated dogs for about 10,000 years and look at the major differences among dog breeds. Humans on the other hand have been naturally selected for 50,000 years when our first ancestors left africa. This isn't to suggest one race is better or worse. But the beginning of knowledge is calling things by there proper names. We can't solve the racial problems throughout the world if we deny that race even exists.