Probably partly the complexity. Your stats affect its stats and they often had to fix odd issues with that. And the fact, as mentioned in other posts, they want jobs to be simple to understand and there are players that don't understand pet hotbars - the same reason they probably moved scholar's pet control to normal hotbar actions. Their pet doesn't attack either and is virtually doing the same thing that Sage does where its attacks do an auto heal on the Kardia partner.
I believe they are replicating the idea from old FF games where the summons were only temporary. They did their attack, then left. However, they've also made this imbue you with their element aether.
What we need is a Proto-Carbuncle summon.
https://static.wikia.nocookie.net/fi...-Carbuncle.png