What is it the military is filtering out?
They stay sealed until it is time to use them. They use them to filter out chemicals from the outside in. If they were worn like regular clothing (hunting clothes) or left outin the airthe carbon would adsorb human "stink" particles and other airborn particlesand after a short time be ineffective against chemicals.
Why do they destroy a garment? Or is it what's inside a garment they are destroying?
They destroy the garment after it has been usedbecasue reactivating it would destroy it any way becasue of the temperatures needed. Also if they had a bunch of used garments laying around i suspect that it would be very possible a soldier get issued used chemical suits that are pastits useful service life.