It's more complicated than that though - from what I understand, the armorie is not like your inventory or retainers in that you're not simply storing it away in a database, when you store something in your armorie it's actually literally deleting that item completely, replacing it instead with a flag to restore it when you remove it (this is why it resets all customization to an item like spiritbond and dye to default values, as it's restoring the item from scratch).

Hence by greatly restricting what items can be stored in this way, reduces the chances of bugs and glitches resulting in items being lost forever due to the flag being corrupted when storing or removing an item from it.

It's just one of those quirks of how the game is programmed sadly.