It's because the Calamity Salvagers were originally meant to be nothing more than an inventory overflow for Legacy players transferring their 1.0 character data into ARR - 1.0 had a larger inventory (200 slots by default instead of 100) so there needed to be somewhere to stick the extra stuff 1.0 players inevitably would have had when starting as their old characters (hence why they're called 'Calamity Salvagers').
Then SE started added extra functions to them, so they now play the part of special vendors, such as recovering special quested gear you may have accidentally thrown away or lost. They're not meant to be a backup for lazy inventory management but more a failsafe id you accidentally lose or misplace the item that you cannot regain anywhere else.
Because the Armorie is not standard item storage at all like your retainer or chocobo saddlebag. How the Armorie works is it literally deletes the gear stored in it, but flags the item to be restored when you want to retrieve it (this is why it resets customizations like Company logos or Spiritbond, it's literally recreating that item from scratch as it's default state). Hence in order to reduce the risk of issues being caused by having too many items stored in it resulting in lost items, SE limits it to only special gear you cannot get anywhere else like from the Mogstation or from special events - AF gear other than the original level 45-50 AF1 set can be gained through simply buying it with the revelant currency from the revelant NPC vendor so as a last resort for lack of storage you're meant to throw the AF away and just buy another set later on. But that's not possible with AF1 so hence it's allowed to be stored in the Armorie.
Using the Armorie for the glamour dresser is simply a means of streamlining things so you don't have to keep going back to the Armorie to retrieve items stored in it to add to glamour plates - it's not meant to be used for all glamour plates, that's pretty clear. I don't see SE changing this I'm afraid.