Why is culture even a thing?

If we weren't group thinking tag-alongs, we'd have way more languages in the world than we already have, and communication would be that much more difficult.

Because it has to be

Not odd.  That cultural extinction is accompanied by a cultural assimilation. We are left with fewer cultures, sure, but those remaining grow ever larger in population and influence.

HP Lovecraft had an opinion for everything:

The fact is, my instinctive loyalties and area of interest seem to follow cultural rather than biological lines…a tendency directly opposed to the Nazi tribal ideal. Undeniably, my own blood kinfolk on the continent interest me less than my cultural kinfolk—whose blood diverges sharply from my own as the stream recedes in time. The northern nations—biologically akin to me—seem foreign and of minor interest; whilst France, Italy, and Greece—the successive cultural precursors of the Anglo-Norman civilisation around me—seem close, ancestral, and of vital personal interest. To me the Roman Empire will always seem the central incident of human history—and this perspective cannot but colour (both consciously and unconsciously) my national interests and literary appreciations in connexion with the modern world. Incidentally—this perspective was quite typical of the 18th century, to which I am so inextricably bound. The conflicting inclinations and tastes of a composite civilisation—where race and artistic-intellectual heritage spring from different sources—form a curious study. Conscious, objective interests tend to follow the line of culture rather than of race; but inward mental and emotional processes (ethical concepts and compulsions, social-political preferences, trends of imagination, modes of every-day living, &c) gravitate toward the line of race. An Anglo-American can talk art and history and philosophy with a Frenchman better than with a German…yet his unconscious habits and outlook and way of life make him vastly closer to the German in practical, everyday matters.
>from a letter written June 13, 1936

Yeah, because Lovecraft's credentials as an anthropologist or sociologist are impeccable.

Quoting him is like telling someone about the nature of neurology by quoting the musings of your plumber.

Actually, he was one of the worst writers in the history of English when it comes to prose. His genius was the way he turned his absurd psychological issues into horror. And his world-building collaborations with other writers. But no one with any taste has ever praised his actual writing.