A lot of that is mirrored by unexplained fantasy stuff. How do Dwarves live so long? Where do gods come from? Why does magic exist?
This varies from author to author of course. The PHB has a ton of it though.
Is the few hundred years the dwarves live a long time? Do readers generally assume humans live, well humanish lifespans until told otherwise? Should they? If they didn't should Aragorn remarking on his age, for example, have surprised anyone?
I'm betting if we took a survey or tested it, almost every reader assumes just about everything they are reading about works similar to IRL or their previous exposure to it until told otherwise. Is it even possible to make sense of a reading without grounding the words and basic interactions to what we know if not told otherwise?