Just curious, why not?
Because rules don't need to meticulously detail how a character reacts to something scary or potentially dangerous for them to still act appropriately when such things present themselves.
Games have tried, mind, to get that deep. You and I don't know those games by name for a reason.
Mechanics that work against the feel want are actively bad.
The lack of a mechanic doesn't prevent the feel from occurring, however, as these are roleplaying games and as such, unless you're going to mechanize every single facet of what it takes to roleplay as a particular character, theres going to be things the system doesn't give you mechanics for because they simply don't need to exist in the first place.
Keep in mind, there are a lot of things players expect to be able to do in these games, and if you take this logic to its actual conclusion, those things are not possible.
And for the great bulk of games, that includes the very idea of telling stories. No story mechanics? Sorry, game doesn't support it. Want to introduce a plot twist? Oops, no twist mechanics.
Making arbitrary lines where we let one side of a GM screen actually roleplay while the other can stick it is ridiculous.
The constructed reality will be determined not by the reality of our world, but by the reality created by the rules
See above.
And if you as a participant in those games don't like that
Interesting, considering what you go on to say.
Throwing a temper tantrum about metagaming or how the character wouldn't do that is just bad GMing.
I was actually referring to GMs abusing this logic to screw over players, not the other way around, but it ultimately doesn't matter.
There are a lot of games that don't have an explicit section in the rules detailing how Lava works.
That doesn't mean any GM who isn't a complete novice (and a pushover) isn't going to immediately rule you dead if you dive in to a pool of it.
And thats a very simple example of the problems created by this logic. It gets more complex when you do things that start violating the fantasy being portrayed with the flimsy excuse of "no rules".
If a Barbarian is blocking off an area so his Wizard friend can cast spells, I don't care what system it is or what rules it has. You don't phase enemies through the Barbarian, and you definitely don't throw out this flimsy nonsense as a defense of that.
Because thats not just "playing the game". Thats you looking at two players coordinating their efforts and working as a team to win their encounter, and you're telling them to take that highly desirable playstyle and shove it up their rear ends.
Thats how you start making toxic tables that just revolve around screwing each other over.
And to think you have the audacity to call that "Good GMing".