The GM's construct of the "world," or the "fiction," or the "milieu," or whatever you want to call it, is just that---a construct. At what point is the construct "complete" enough for it to feel "real" to you?
Does there have to be detailed background information for every point or line drawn on the map of the world? Every city? Does every town need to have 20 fully realized NPCs before it will feel "real" to you? Do all 20 NPCs need to have fully realized daily schedules so you can roll on random tables to see if the PCs encounter them?
Does every nation-state in the world have to have a detailed 3,000 year history, with a list of kings, queens, regents before it will feel "real"?
And if not, what components of the construct do you decide is privileged / has primacy in making it "feel real" to you?
The thing of it is, a GM has to constantly generate off-the-cuff / in-the-moment "stuff to add to the fictional construct" no matter how much of the construct is prefabricated.
There's constant additions as the players interact with things in the world that simply didn't exist until the very moment the player says, "I look at / touch / act on X." It doesn't matter how much prefabrication happens beforehand, these situations still arise in every single moment of every single game session.
Yet somehow, a GM having to constantly make these spur-of-the-moment additions to the prefabricated construct don't make gameplay / the gameworld trite---but using a ruleset that systematically enables these additions coherently does?