Because the loser of the video game would rather resort to actual war rather than accept the consequence of losing a war because they lost the video game. This is also why, more relevantly to serious foreign policy efforts, international systems of peaceful resolution like the International Court of Justice haven’t replaced actual war as a means of resolving disputes.
IIRC Bones (McCoy) calls it "Monstrous"
And that's not even getting into war as a racket, its utility for propaganda and cultural control, boosting the economy and funding the military industrial complex.
It is assuming the characteristics of a contest, especially for wars fought in times of abundance.
Roman and Persian generals would scratch their heads at the precision with which we try to avoid collateral damage, and even soldiers on the field nowadays have a band on their arm identifiying which team are they with (it's yellow for Ukraine and Red for Russia).
Even going back to the days of Kosovo and Bosnia that was unfathomable.
Who is the “you” that can establish and enforce this?
If such a party exists, they have the power to defeat either of the nominal parties in war, and the ability to dictate results to them, and obviously they care enough about avoiding war to dedicate the resources to enforcing a resolution, so they have already rendered the war unnecessary: why would they not just decide the result and enforce that decision rather than letting a videogame do the deciding for them?