That is a good question, and there are many answers. Some of the equipment is being stored nominally for emergencies (an unanticipated war). There are a lot of things that cannot be adequately deweaponized for reuse in civilian applications. Some warships and aircraft are sold to foreign governments, however, both a hull and an airframe have practical usage limits. Once you reach those limits, the vessel can no longer be used for its intended purposes safely.
Speaking of safety, there are rules in effect now that prohibit the practical refitting of older aircraft from being reused for commercial (or even private) applications, such as requirements for emergency locator beacons and anti-collision equipment (there may be waivers for museum-type planes such as those used for airshows...) which carry a hefty price tag for purchase, installation and maintenance.
Disposing of things built more than thirty years ago can be a problem because of some of the materials used in the manufacture (think PCBs, lead, and asbestos) and deconstructing these hulls is prohibitively expensive.
For that same reason, you will find that there are shipping container graveyards in many coastal areas: they are put out to rust because they are unsuitable for their intended purpose, and/or too expensive to ship empty back to their countries of origin and recycling is not profitable.
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