This may sound very cynical, but i have a suspicion this is not high on their priority list, despite most competitors offering similar deals, because unlike games like WoW and Gw2 the only way we have to increase our bagslots given by the game is to rent it for real money.

The two aforementioned games have craftable bags and such therefore not creating an incentive to limit the players bag space. FFxiv on the other hand has no such feature. If a player run out of the already quite large bagspace, they have to either buy more or suck it up.

Mogstation purchased gear only takes away even more slots as it cant be put in armoire/be repurchased. (to my knowledge at least, i have not bought anything)
In stormblood they say they will increase bagslots. This will solve the issue temporarily. No matter what the game is, players will always find a way to run out.