NATO was formed to counter the threat of the Soviet Union which no longer exists. Russia is nothing like the same threat to the US that the Soviet Union was. The US is now more concerned about China.

Europe was economically and militarily important globally than it now is. Therefore defending it is less important to the US than it used to be, while Asia has become more important to the US.

Russia is not even anything like the same threat to Europe that the Soviet Union was. It has been further weakened by the Ukraine war and it will take many years to recover.

As for nukes, there are two acknowledged nuclear powers in western Europe.