Then only one single crazy national-leader has to not actually demilitarize, and he can then conquer anyone who truly did demilitarize. Which means that the logical thing for anyone is not to demilitarize. Which means that the overall result is we don't demilitarize.

Breaking that equilibrium would require a militarized worldwide government, which also would likely e.g. lack the ability to respect current nations' differing cultures (to summarize the vast body of ways the very concept is ... unnerving at best).