Your expectations are way too high. Players are already primed to play along with whatever characters/story you impose; they aren't actively looking for flaws, so you can fudge a lot of it. It just needs to be acceptibly believable, not correct. And since you're the DM, you can usually coerce things to go towards the areas that are fleshed out, or the subjects you know/interested in. And when players go off rails, they tend to put a lot more effort into solving the logical inconsistencies, and relieve that burden from you. In fact, you usually have to start poking holes in their idea so you can punish them to a certain degree for gaming (so they don't get too arrogant about it)

Most npcs don't need to be fleshed out at all, since they're only there to drive setting/plot. If players interact with non-important npcs too much, you just think of a character from some other story/movie and steal their behavior/attitude. And in fact original dnd would have basically omitted them altogether (and roleplaying in general) since it was focused on the gameplay and dungeon crawling aspects themselves.

You need to tune things to the various player's tastes to a degree, but for the most part the players do half the work for you, and many components of the world no one cares about, and doesn't require expansion. You're just tying everything together, and maintaining sanity in the world.