You absolutely know how long it can be expected to last. Explosives expire, at which point they cannot be reliably trusted to explode when they are supposed to or to not explode when they aren't supposed to

Now, if you want to keep a warehouse of kalishnakovs in every city just in case, those won't go bad assuming right conditions and that theyre made of durable materials (no plastic). But they may be outdated by the time they're needed.

Keeping overcapacity for weapons of war is a balancing act. You don't want to be caught with your pants down, but you also dont want to be sending boats full of tanks that nobody wants to every conflict lest you have to shut the plant down.