Because there is currently little to no incentive to actually open the cages full of gremlins containing enemies that are not boxes that do not contain heat crystals.
Or maybe have it so that 2 treasure cages spawn, with 1 of the gremlin cages containing a 2nd key that drops by a hidden switch or something? But reward the players for opening the cages again.
Keep in mind that the gremlins themselves drop loot.