Because you're not thinking of this as rules for a game that happen to take place in an imaginary world.
Instead, you are trying to say that the rules define this imaginary world.
In other words, in your conception, a 20th level fighter says, "Hey, I don't have time to wait for feather fall, so I'll just jump down that 200' chasm since I know it won't kill me, and I don't feel pain, or anything like that."
It's a completely different understanding of, um, playing D&D. It's okay, but it's different.
I think you've correctly identified the source of much of the divide in this thread. I don't see the split as perfectly distinct, however, because personally I use both approaches simultaneously for different elements of the game world.
I assume the game world largely works like the real world when it comes to the elements they share. For those elements the rules are an abstraction to be used when they are useful for modelling action resolution in a fun and believable way, and modified on the fly when they aren't. (Although to ensure fun, in practice the on-the-fly modifications are almost always to let the PCs do something the rules don't cover, rather than going outside the rules to stymie the PCs.) So the rules don't define the game world at my table with respect to elements shared with the real world.
But the game world has many elements (e.g. magic) that aren't shared with the real world. I assume these elements
are defined by the rules, because that's the common material the DM and the players have to work with. So seeing as how characters with Pact Magic have spells that last longer than it takes to recover the slot used to cast them, I'm fine deducing from the rules that in the game world Pact Magic casters routinely cast such spells before resting.
Hex has an extra step to take since usefully casting it ahead of time requires both a valid target and that target's death, so I'm also fine with deducing that animal sacrifice is a routine part of using Pact Magic in the game world. (Given the association of curses and animal sacrifice in literature, I'm actually
thrilled with maintaining that connection in the game world.)
Of course, not all inferences that can be drawn from the rules about how magic works in the game world are going to be positive, and which ones are positive and which ones are negative is entirely subjective. If I didn't like an implied link between curses and animal sacrifice in my game world, I wouldn't use it, and would add some element to the game world to explain why Pact Magic casters don't sacrifice animals to allow them to pre-cast
Hex.
As an aside, I do think there needs to be
some shared understanding between the DM and the players regarding how the fantastical elements of the setting work. Extrapolating from the game rules is one approach, but detailed setting-specfic lore also works. Strong tropes might also be a workable substitute, although the large variation in tropes from fictional work to fictional work makes it tricky without an explicit discussion of which tropes define a particular setting.
It seems like some posters in this thread are definitely using this last approach, and their objection to hexing chickens stems from it violating the (possibly unspecified) set of tropes they're using to define how magic works in their world. Where it gets tricky is when a player doesn't understand what the defining tropes are, and thus has no way to know whether the DM will consider hexing a chicken to be wildly in-theme or problematically silly. Politely asking first is therefore undoubtedly wise.