Frustrating Design

[aesteval]

That is not the design reason, it is the design choice.
They chose to have it act like a retainer inventory. Why though? The answer to the question is what I call the design reason.

It's increased storage space for the player without a persistent increase in network traffic that apparently comes from regular storage. Check live letter's / interviews from around Stormblood launch regarding increases in inventory space and the limitations that the dev team encountered.

Granted this will likely lead to questions about why inventory and network traffic functions as it does in the first place. I'll let someone more familiar with the 1.x to 2.0 transition field that.