The Limits of Fictional World-Building

In the first scene of the movie Valerian and the City of a Thousand Planets, there’s a handshake between astronauts—hailing from America and China—as one spaceship docks to a larger space station. Subsequently, as the years roll by, more nationalities gain space-faring capabilities, and repeat the same gesture of friendship. Within two centuries, the first aliens make contact at the space station, and soon it becomes of port of call for all interstellar travelers, many of whom end up staying.
It’s a fun premise, but I was dismayed at the sci-fi trope1 that this sequence bluntly contrasts, from the rich diversity of the human race and its many countries and cultures, to the inevitably monocultural alien races which are supposed to represent a planet, star system, or sometimes galaxy. If humanity was portrayed as an alien species ourselves, we’d only portrayed with a singular defining characteristic—something stereotypical like propensity for violence or ability to love—otherwise being completely substitutable.
Valerian is hardly the only offender; both Star Wars and Star Trek have used their alien races as simple caricatures for decades, where each alien species is culturally equivalent to a historical nation state. Nor is it confined to just science fiction either; fantasy also has this annoying habit of reducing non-human races to a handful of characteristic and racial stereotypes, where the orcs are murderous and the dwarves stay greedy for shiny metals.
In fact, the only literary work I can think of that really tries to flesh out its world is J.R.R. Tolkien and his Middle-Earth; he invented different factions and motivations for his races of humans, elves, dwarves, and orcs. It helped that he was a linguist, and that he constructed his languages with an heavy emphasis on their genesis, which compelled him to weave a complete accompanying history and mythology. Of course, it’s understandable why most authors and storytellers don’t build their worlds with the same rigor: Tolkien spent of his life inventing his languages and the history behind them. Besides, if the Silmarillion is any indication, world-building may be a necessary ingredient to enrich fantastical stories, the actual details of the world are a bit dryer and a harder read2.
I do wonder, though, whether this kind of gross generalization of unfamiliar—or literally alien—worlds is a way to keep the audience focused on the plot and not get distasted by minutiae of some fictional world expansion. Faux history nerding aside, the purpose of fictional worlds is usually to provide context for the main story, and the vast majority of stories don’t want the background to be its own character. World building is sometimes best done as hints and subtle cues, a technique that has been especially refined with video game narratives. If this is the case, then a world that is barely fleshed out is borne out of design; it’s just what the audience wants.