The problem is the precedent that sets. Russia launches a nuke on Ukraine, and there are no repercussions. That will teach every nuclear-armed state that they can freely nuke non-nuclear-armed states without consequence. But then what happens when somebody makes a mistake? China nukes Japan, but maybe Japan had a secret nuclear program and actually does have a retaliatory capability and nukes China back? Or China invades Taiwan (doesn't even have to nuke it), but the U.S. decides that the loss of Taiwanese semiconductor is actually an unacceptable red line and nukes the invasion fleet? Pakistan nukes India, but China misjudges the trajectory of the nuke and thinks it's actually under attack. Israel nukes Iran, but winds carry the fallout over Pakistan and India.

Game theory works when players know the payout matrix. When the assumed payout matrix is shown to be false, you get very chaotic, almost random results, because you can't assume that your opponents will correctly choose the rational choice. With WMDs, the consequences of that can be deadly. That's why both nuclear proliferation and "limited" nuclear war are such fraught choices, and why the major nuclear powers have worked so hard to avoid them. They've run the game theoretic simulations and understand that it doesn't lead anywhere good.