I can't say that I understand this. If the rules don't define the imaginary world, what does?
There are more things in heaven and Earth, Jaelis,
Than are dreamt of in your philosophy.
I never thought I'd have to explicate this, but of course the rules cannot fully define the imaginary world. After all, we don't have rules that fully define our own world that we live in, and even attempting to put such "rules" down would require as many reams of paper as there are atoms in the universe.
So it would seem hubris to say that there are rules that define this imaginary world, right? After all, what rules in the 5e define what a shopkeeper in a town 40 leauges away is doing when you're in a dungeon? What rules define the chance of an NPC that you've never met dying during childbirth?
What rules define the tactics that the monsters are going to use on you?
If a fighter knows he can survive jumping down a cliff, why wouldn't he?
Because, assuming that this in some way resembles our own world, the fighter doesn't know that. Instead, we use a "game mechanic" called hit points to include "luck" and other intangibles, so that if our superheroic fighter accidentally falls 200', we can come up with a fictional reason for the survival.
But again, these game rules aren't reality, nor can they be. Nor can they "model" or "simulate" a world. So demanding that they exist, and then demanding that the world follows these game rules, and then saying that this is the essence of the world and how it operates ... well, that's crazy!
To me, at least. YMMV. Play as you want! Jump down those chasms.