Spies exist. You never are sure how many the enemy has or what they are leaking - even if you do learn, that is a snapshot in time: tomorrow they may get another spy and then you are back. As such you are risking the enemy learns the nukes don't exist and then the whole thing fails.

We now have treaties to inspect each other's nukes. So unlike other things we have a good idea of what how many exist and are working - or at least a floor on the number (that is either side may decide to risk the treaty with secret nukes elsewhere. Between the potential of the program being discovered and the costs I doubt it, but I must allow this for completeness). As such it is generally believed that Russia's Nukes are in mostly in good shape - those who should know all seem to be acting like they are.