It's not impossible but it's more complicated than stacking items like potions, because data.

You can stack potions because every potions are the same, so the game only need to store the item ID and the size of the stack.
But each equipment are different since they all have a durability, spiritbond, dye ID and glamour ID. You can't stack 10 "Anemos hat" because the game still needs to store 10 different durability, 10 different dye, etc.
Of course we don't care about durability and spiritbond for glamour gear, but they're equipments so they work like all equipments.
We can imagine a feature, kind of similar to the armoire, where you can stack equipments but lose any dye/glamour/spiritbond, but it would be a lot of work for a very special case.
Plus, I'm basing my explanation on the data we see as players. Items may have some more data tied to them on the server.

For the lockbox opening UI specifically, the simplest addition would be a "Throw away" button.