They can't keep them armed on the ground? Or does OSHA kick up a ruckus?
Combo of things. 1) Once you arm them, then you have to put (armed) guards* on them 24/7, 2) Sensitive electronics and stuff degrade in long term exposure to weather, 3) There was no point. It wasn't that there wasn't an air threat to the US. It was that there was no Threat. Period.
% 4) what Larry said. If you are flying out to Nellis to train, you don't want a bunch of warshots hanging off the plane (and real bullets in the guns), what's the point again of loading it up, just to unload it when you get there. And then there's the turn-in of unused ammo. Fark that paperwork. And even if you are not flying to Nellis to train, even if it's just to burn holes in the sky to keep your hours up, eventually some dumbass would accidentally let a Sidewinder go flying ....right into a 747 filled with orphans and pregnant nuns.
Nope, keep the missiles and bullets in the ammo bunker, where they can't hurt anyone, nor get anyone in trouble.
# * - And guard duty is boooooooooooorrrrrrrrrrrrrrriiiiiiiiiiiiiiiiiiinnnnnnnnnnnnnnnnnnnggggggggggggggg. Until it's not. And then you are once again playing catchup.
% - There was not a threat to the US. The USSR and Warsaw Pact had ceased to exist 10 years prior. We weren't even pointing missiles/ICBM at each other, but had agreed to aim them at the Sargrasso Sea. Wargames were against the "Krasnovians" who were Iraqi/Soviets. In fact, we were still trying to figure out what, if any, doctrine and capabilities our "adversaries" had. If we could find an adversary. We were doing things like Mogadishu, Bosnia, and Kosovo. Peacekeeping-style missions, and those were going to be the missions of the future. Not killing the enemy and breaking his stuff, but getting all-sides to gather around the camp fire and sing kumbaya.
# - My last company commander about had kittens when he found out I had busted into the sealed containers with the bayonets and issued out 1 each to my soldiers prior to an FTX where we were going to practice mine detection methods. Kitten's because OMG!!!1111 someone could have gotten hurt with those !!!!!