Chest Monster/WMG

An Inexplicable Treasure Chest is the cast off shell of a Chest Monster.
Which is why they're everywhere, and why nobody seems to have the job of making them. Some alterations for locks; and presto! Instant container. Pity it still looks like something trying to kill you.
 * So Chests actually look like Mimics.
 * In addition, mimic spores coalesce around items that adventures drop because their screen is full. If nobody comes around long enough for them to feed on, they die or cast off the shell and find a different spot; leaving a chest with a single item in it.

Alternatively, any Inexplicable Treasure Chest that doesn't kill you is simply a non-aggressive Chest Monster.
The "core" of the Chest Monster is actually the item inside. They use adventurers to spread their spores around; and reform their chest shell when they get dropped.
 * They die if it's a single-use item, but this happens in nature all the time. As long as more are spread than are consumed, it works.