Expired Goods

Magic items are, by definition, magical, and as such are preternaturally durable. It wouldn’t at all do to discover a wondrous magical sword jutting from a medium-sized boulder in a courtyard that was rusted beyond use after a couple decades of exposure to the elements. Potions stretch this idea somewhat, as they’re a substance you put inside your body. Even straight water in a flash gets kinda icky after a few weeks, let alone centuries in a musty dungeon. Even if the potion itself didn’t degrade or go off, there would be ample time for bacteria to grow and fester inside the container. Drinking a potion should include a not-insignificant chance of horrible food poisoning.