"The Planet has power? What can the Planet by itself do? No, the Planet itself has no power. The Lifestream and Holy do. For all other threats, the Planet created the Weapons. "
My point is simply you seem to see them as separate entities.
So the planet made it's main defence against it's to date most dangerous threat, something that depends on there being a Cetra present, with the materia.
All other threats, such as fish, sand worms, birds, and the odd island village are to be handled by giant killing machines. Which were coincidentally built at the time of the first crash, and resummoned prior to the second.
