Race refers to phenotypically distinct branches of the human species. The concept has sociological implications, but the concept itself refers to human biology, which is constructed genetically, not socially. It is possible for an individual to grasp the concept of race by way of his own observation of groups of individuals who possess visibly distinctive traits, so the existence of race does not require social validation, let alone social construction.
Common observable phenotypes among populations are explained by genetics, not society. But the concept of race has meaning because of the observable manifestations of genetic variations, not the percentage of genetic dissimilarity. Aren't humans 98% chimpanzee genetically? You could break down a human being to all his chemical components that might exist in a laboratory, but those chemicals wouldn't constitute a human, wouldn't distinguish a human conceptually from other existents. The chemistry of humans underlies but does not define humans.
The essence of race is phenotypical variation, not the genotypical variation that underlies it. Phenotypes per se logically can't be sociological. If you are going to call common observable phenotypes among populations such as skin color "social constructions," even though it is possible to observe them outside of a social context because they exist biologically, then you have rendered the term "social construct" meaningless insofar as all concepts would be "social constructs," including the universe itself.
So to be consistent, if I were an astronomer attempting to describe Uranus, you would have to try to undermine the information I'm giving you by telling me that Uranus and all planets are just social constructs. There is some disagreement among astronomers on whether Pluto qualifies as a planet, but there is no disagreement on whether "planet" is a valid concept that describes actual objects in space and differentiates them from other objects, such as comets and stars.
Concepts that reference existents can't be referring to things that are socially constructed. Social constructs could only refer to such things as laws, mores, customs, etc. -- things that don't actually exist in reality, only as concepts derived from social interactions. Anti-discrimination laws are social constructs, but in order for any laws against racial discrimination to be enforceable, races must be presumed to exist. Otherwise, one accused of discrimination could prove his innocence by demonstrating that: He couldn't have possibly discriminated based on race since races don't exist; and social constructs don't have any legal rights or standing to claim discrimination anyway (no "black person" or "white person" who might claim discrimination actually exists except as a social construct, he could argue).
Do you see why the distinction between purely social concepts and actual existents in reality is useful? It's a distinction the Orwellian abusers of language want to obliterate in the name of prescribing an egalitarianism for the human species that nature failed to produce.