Simple enough - nuclear warheads wear out. They get stale, They cease to function. They must be replaced.
The US had millions of rounds of 50 caliber machine gun ammo left after WWII - enough to last through all the wars since then. Is that stock still around? No - it was destroyed and replaced with fresh ammo. Same deal with nukes - they are not a solid block of stone, so they do no last forever. No complex weapon does.