As a programmer, you aren't giving every character a tick sheet of every glam possible in the game. It's way too massive. From the server-end you want to build a list of what they have, not what is possible. In-fact this is done in a lot of games today. 6+ year old players have longer lists, but 1 month players don't and that's how it's funded.

Inventory in an MMO is hard, this isn't an offline game where they can just store everything you have in a save file on your PC and load it up like lighting a Christmas tree. The limitation of your inventory you can carry with everywhere is basically capacity driven, you only have so much bandwidth to allocate to every person but you want to do it wisely so you aren't cutting game quality. Those 100 slots have to follow you everywhere you go in the game, including the instance servers. That's a hit on the servers as much as you casting a spell. If you gave millions of people 100 more inventory slots overnight, you can bet the servers won't be too happy without major changes, so what do you do?

So this is obviously this is why the chocobo saddlebag was created, now we have inventory that doesn't have to follow you into instance servers because we have more world server capacity and it won't cut down on instance server quality which track the player more often than you out and about in The Lochs. Then we have the glamour closet, where it's technically inventory, but it's only available in the towns that obviously more server resources dedicated to. Plus you are going to a consolidated instance when going to the hotel, so it cuts you out of side-by-side player interaction and into your own space to allow this expansion without hindering game performance.

You notice why you can't open equipment coffers outside of towns? This is related to this as well. The instance servers don't hold the loot inside that chest you won in E12S or that job jump chest of gear you bought from the mogstation.

It's likely way easier to add more glamour chest capacity than anything else in this game, it depends on those hotel instance cutouts which they have improved quite a bit over time like allowing you to still queue for stuff.