Because every time it is brought up it is easily demonstrated that nobody actually agreed to defend Ukraine. They each agreed not to invade Ukraine.

The only assurance of assistance built into the agreement is that parties would complain to the UN security council.

The Obama administration let Russia violate the agreement when they took Crimea. Putin's justification that Ukraine isn't a nation but a historically Russian territory only works if Russia pretends it was never a signatory, since recognition of Ukraine as an independent and sovereign nation was a part of it.

In short, pieces of paper do not trump the fear of nuclear weapons.

So? If nuclear war is always an intolerable choice for America, then America's nuclear weapons will never be used in any case, and any aggressor who can plausibly threaten nuclear war is free to seize anything from America with that threat.

If nuclear war isn't always intolerable, then, well, we've eliminated the premise off the upthread argument about Ukraine, and are in a different discussion.

Had Russia invaded a NATO country, we still would not have launched first, but rather used conventional weapons, given the fallout (literal and metaphorical), but actually using them to prevent greater numbers of casualties from protracted war is not fully off the table, so to speak.