Not post-apocalyptic. Pose-post-apocalyptic. As in, there was a collapse and people were living bleak lives in the ruins of that civilization, but they have all died off at this point and maybe just some of the elderly elves or dwarves actually remember the empire of old. To everyone else, those stories are mytholological... except in that you can sometimes find a intact ruin from that age and wander into it to find some slippers of spider walking or whatevs.
So, in those worlds you play in, why is it that you find artifacts and relics in dungeons, instead of just spending your gold on whatever fashionable item is being mass produced by the bustling economy of the golden age you're in the midst of?
I'm kind of a god of remembering where I learned things from (or maybe that's just part of autism...) but there are limits to that. At this point I know how many towns most likely exist on the periphery of any one city of a certain size and how many thorps are spread around between those and how much wilderness disrupts that orderly pattern. I can't spit out ideas fast enough to make every location interesting and noteworthy; nobody can do that, which is why narrative structure is always so ignorant of how many settlements there are in a country. A story cares about the most interesting locations and maybe a small selection of other locations just for establishing how we got to the interesting parts. That's not because people in the real world hadn't fleshed out their little villages enough, it's because there's only so much you can expect anyone to remember, or pay attention to in the first place.
Like, some time, try to actually draw a complex that occupies 1 square mile and then describe the purpose and contents of every room. 1 mile2 was way too much. If you're absurdly dedicated then you managed to fill the entire space, but you probably have to guess at what's in the 37th kitchen, unless you made every kitchen identical and boring. I do some programming as a hobby (with a little proper training,) so I routinely create things that are too big to keep in my head all at once. As soon as you start using a system to organize things it becomes really easy to go past what you can remember.
Narratives are different though. If you're making a story then it's got an order to it, you know which parts follow from each other, and the twists that don't follow from what's happened so far are probably memorable moments.
I don't know what kind of focus you take with fleshing things out, but if the narrative is the bones, you'll run out of room for remembering flesh pretty quick.