They need to do that too, but just look at the numbers;
The Armoury Chest has 25 slots per equipment type, so we can have 25 weapons for 25 unique Gear Sets.
There are 13 Jobs in the game, all using unique weapons/Gear Sets.
There are 3 Gathering Classes, and 8 Crafting Classes, again, all using unique weapons/Gear Sets.
That's a total of potentially 24 unique weapons/Gear Sets.
Now, you'll always have a weapon equipped, so there are typically 2 spaces left in the Armoury Chest space, but that's it. For other slots this is less of an issue, since a lot of gear tends to be shared (especially with Gatherers/Crafters).
This is a problem. If they introduce an entirely new set of gear for a specific event, we simply don't have the Armoury Chest space to support that... If you need a Morale weapon for PvP and a regular weapon for PvE, we can't fit that. Likewise if we need a specific Palace of the Dead weapon... This is definitely going to be an issue when new Jobs are added (and we got confirmation new Jobs are coming with 4.0), since we only have space in the Armoury Chest for an additional 2 Jobs, if they add any more we're going to need to make choices on which Jobs/Classes we bother with, and in a game where "You can play all the things on one character!" is the main selling point, that seems pretty sh*tty to me. They really need to just give us a second Armoury Chest already, or revamp how the current one works, because right now some slots are just wasting space; You'd be hard pressed to carry 25 different off-hand tools for example. Looking at my character, I've got 10 free slots in that section, and 3 shields I definitely don't need to be carrying around. Waist and the accessory slots (minus Rings) are also all fairly wasteful. While leveling that can be a different story, but I don't mind inventory issues while leveling that much... My Armoury Chest has a lot of wasted space in it though, because for some reason every equipment type can only have 25 dedicated slots... Add 3 new Jobs and I'll need 26 dedicated slots for weapons so 1 Job will be left out, meanwhile my off-hand slot has plenty of free space that could have accommodated that Job... It's silly...
TL;DR - The Armoury Chest has 25 slots per equipment type, for a total of 300 spaces over all gear, but it's forced that you can only carry 25 weapons with it, for example. If you play everything, the weapon slot is very close to full, limiting how many additional sets we can have for new Jobs/event specific gear sets. Meanwhile, slots like the off-hand slot simply are not using all 25 of those slots. They either need to just straight up give us a second Armoury Chest (+300 inventory split across +25 equipment type, wasting more space on certain slots), or they need to adjust the current Armoury Chest to cut down that waste.