Frustrating Design

[Dirich]

Because the chocobo bags act like a retainer's inventory. You can't use items when interacting with a retainer, same for the chocobo.

Not saying I agree with it, just explaining why.

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.