shouldn't the game experience to be as close to that as possible?
No, its a game. Let people know exactly how much things change, but hide the exact thresholds for various conditions (eg you don't know if the player will attack you at -4 relations or -6 relations or -8 relations). Like in Civ4.
The purpose of the UI is to inform the player as to the affects of their actions, not to hide information from them so as to make it "realistic".
Will it make a large or a small difference? try it and find out. play the game
Trial and error is a terrible way to learn. Why should I have to experience Earth's history several times in order to figure out how the diplomacy engine works?
The process can be transparent without being numerical.
Maybe... but making it numerical is so much simpler, and does a better job of conveying info to the player. I see no advantage to abandoning numerical indicators.
It is good for the player to know that attacking their friend makes them 3x as angry as does refusing their tribute, or that open borders matter 1/5 as much as having once razed one of their cities. This allows you to actually consider a rational diplomatic strategy. you can't easily do this without numbers. It also lets you observe things like "cooling off", where the impact of an action can decline over time. (ie finite duration grudges). You can't convey this kind of info easily without numbers.
The whole game model is an equation, mostly deterministic; combat is an equation with probabilities.
Being mathematical and theoretically solveable is not a reason to abandon a mechanic.
If the human player can definitively predict the AI response, the human player stops playing the game and starts playing the mechanics
If the human player cannot predict the AI response to a significant effectiv, then we can't engage in effective diplomacy.
You can't definitely predict the AI response in Civ4.
I see no design gain from hiding data from the player, they're fixing something that isn't broken.
There's a reason why people like the BUG mod, which reveals even more of the hidden AI/UI variables to the player.