The Nukes are a deterrence. So it's fair to say US/NATO don't date to attack Russia.
But I believe that's not the only reason why NATO/USA refuse to engage Russia militarily. Because Russia still have significant reserves to be activated in times of war and that does not include general mobilization.
And NATO is not ready for war with Russia. Even their own politicians admitted.
https://www.independent.co.uk/news/world/europe/russia-war-nato-ukraine-b2061001.html
So a NATO/US intervention will risk NATO being attacked and collapsing all together.
That's why I always said, don't underestimate Russia. They are hiding a lot of things, including their military complex's full potential to produce weapons and also current stockpile of weapons including armor and ammo.
Just look at Ukraine now, They are losing badly despite all the so called wonder weapons being supplied to them. Javelin? Stinger? M777? Russia is still gaining ground isn't it?