The point is that almost all of the signatories considered themselves to be immune to a "real war" in their futures at the time they signed. E.g. basically all of the European signatories assumed that the end of the cold war and existence of NATO would ensure the end of any possible threat. Given that assumption, as obviously flawed as it was, signing on to a ban was cheap PR (literally cheap, too, because it meant they could divest those weapons and their delivery mechanisms to reduce defense expenditures).

which is exactly why european countries threatened by russia are starting to withdraw from the treaty, five had recently announced so