

Because a nuclear retaliation by France or the UK against Russia would result in the destruction of France and the UK. Imagine this scenario: Moscow drops a nuclear bomb on a major European city. Do you think Macron would stand up in front of the French people and say: “This means the end for all of us, but we will now take revenge on Russia”?
Strategic nuclear weapons have only one purpose: to protect one’s own existence from other nuclear weapons in a tit-for-tat principle. They are completely unsuitable for supporting allies, because their existence is always considered less relevant than one’s own.
One exception was the Cold War, when the US considered NATO and the Soviets considered the Warsaw Pact to be their quasi-property.





France and the UK did not use nuclear weapons during the Cold War. And the US explicitly and unambiguously extended nuclear deterrence to the entire NATO territory. But that is no longer the case. France and the UK would have more interest in dishonoring the alliance for existential reasons alone, as long as they themselves are not bombed. But from the US perspective, the Europeans were vassal states. An attack on them had to be seen in the US as an attack on its own territory. The same does not apply from a French or British perspective. Anyone who thinks that they would risk their existence for an act of revenge against Russia is living in La-La Land.