We could call people ... people? Do they have to be black or white?
Also people with darker skin due to lots of UV exposure are at LOWER risk of vitamin D deficiency than your average pale skinned person.
So it comes down to genetics. Like it or not. Facts of life.
To say there's "no such thing" as race is incredibly, you know, post-factual. Might as well decide there's "no such thing as sex", just half of humanity is inexplicably infertile.
In medicine you don't get everyone's legs preventively amputated because maybe some people have problems with their legs. In the same way we can't just amputate our dictionary and minds of useful concepts and knowledge like the idea of race, just because some people have problematic interpretations of race.
Also, I don't know what point you're trying to make by implying different base conditions. A "black" person who likes to sunbathe in their garden is also at lower risk of vitamin D deficiency than a "white" person who never leaves their basement. So what? And surely you're aware that at higher latitudes "white" people with darker skin are at greater risk than lighter-skinned "white" people? And "black" people with lighter skin are at lower risk than "black" people with darker skin?
Of course it's down to genetics—at no point have I implied otherwise. The variance in baseline melanin is of course genetic! But, like it or not, "a characteristic is genetic" does not mean "races are real". I suggest you catch up on the subject of human population genetics.
There is no such thing as human race in the biological consensus. Rather, it is an arbitrary social construct present mostly in Western and especially American society, which doesn't even have consistent interpretations across regions or populations.
I mean, that's literally the definition of a race: "a population within a species that is distinct in some way, especially a subspecies".
I'm not sure arguing whether a concept is real or not real makes sense. If a concept can be clearly defined, it's "real".
There is. There is however no clear way to create a line saying "here one race ends and the other race begins", there is a continuous spectrum between Europeans and Africans and Asians. So the way USA tries to put you in discrete bins like "White", "Black", "Latino", "Asian" is unscientific, but the concept that humans whose ancestors came from different parts of the world are different enough for them to be considered separate races is not.
Technically there are more races of humans in Africa than outside Africa since it is so diverse, but USA still groups all of them as "black", its just pure nonsense what they are doing.