The moon was a really complex creation. It had to have the excess security to keep intruders out and whatever was kept there, in there. It also was meant for people to travel to and from for general work and possibly emergency escapes should it fail. The tower is but was meant to be a nod to FF3. The tower in lore was built to receive and distribute the energy generated from the moon. It's also likely the moon was designed to be more than a cell and power source hence it would be way more complex than a tower that isn't in a slow decaying orbit. Although all those clones in the tower had to have been kept somewhere. My theory is the weapons (omega, ultima etc..) may have been designed to trap and transport, primals to the moon via ships as well as receive that energy from it via the tower for conquest. It also sounds like something Garland would like to do if they could.