Yes, aesthetics is the word I was looking for! 40k is one part a 'Hyborean age but for military adventures' (the Hyborean Age let REH tell oriental adventures, Injun Fighting, pirate stories, Latin intrigues, and crusader adventures in one setting) and one part a reskinned Warhammer Old World (Chaos Warriors -> Chaos Space Marines, elves -> Eldar). There are even hints that the Old World might be out there somewhere in the void. it does not even engage with science in the <insert tecnobabble> way, but it has all the visual tropes: spaceships, FTl travel, psi, aliens, city-worlds, declining interstellar empires.
One of my go-to models is rational vs. dramatic fiction. Rational stories are about working out the consequences of a premise, dramatic stories are about whatever is exciting. So in Star Wars travel happens at the speed of the plot, whereas even Tolkien carefully worked out the chronology of Frodo's journey to Mt. Doom on a map, and Heinlein was proud of working out the orbital mechanics for Destination Moon on a roll of butcher's paper, he on one side, Virginia on the other, then checking each other's work.