Because it's the same model that they used in XI, where they charged you an extra $1 for each alt character you had on your account, and those characters each came with 7 AH slots and 50 extra storage spaces. I think I had 5 extra characters on my account, in addition to 3 characters on a second account, by the time I finally quit FFXI. Unlike XIV, it was trivial to transfer stuff to other characters on your account.

They switched up the model a bit for XIV so that people could actually use their extra character slots to develop characters. With retainers, they gave you more market board slots and more than double the storage for twice the price, but a retainer is basically just an XI alt that you don't get to play with, otherwise.