
Originally Posted by
Cilia
Eorzea is (barely) able to fend of Imperial assaults only because they only ever send one legion at a time, and even then it's a very close shave. The VIIth had all but routed the Alliance at Cartenau before the Calamity, and without the PC's intervention (and Hydaelyn's assistance in the endeavor) the XIVth would have won. The only reason the XIIth didn't overrun the Ala Mhigan Resistance was because Zenos wasn't interested in actually winning, and after our escapades in Othard liberating Doma (which took the assistance of both the Confederate pirates and most if not all of the Azim Steppe's Xaela tribes) couldn't call on reinforcements.
(Then again the Bad Future Crystal Exarch G'raha hails from suggested the Alliance could not just hold its own, but actually beat back the Empire.)
Superweapons like the... Weapons are one-of-a-kind prototypes that demand a massive amount of resources for R&D; they can't just throw the things around like mass production units, and even if they could the materials for making one probably cost a fortune. Likewise the G-Warrior is only refurbished Allagan tech; the Ironworks didn't make it, probably don't have the resources to make another, and are just using the captured Imperial maintenance facility in Terncliff to house and repair it since Eorzea more than likely doesn't have somewhere suitable. (Maybe Castrum Marinum, now that it's been cleared out.)
The Empire's got advanced tech, but it's best stuff is one-off superpowered prototypes they can't regularly deploy. It's day to day units, such as Magitek Reapers and Predators, can be defeated with the right strategies (Raubahn mentions having to learn how to deal with them late in Stormblood) or enough firepower (see the Legacy ending / ARR opening cinematic where a BLM blasts a Reaper with a fireball and destroys it).
As advanced as the Empire's tech is, it's a bit tough to fully overcome people who can pull meteors out of thin air. Plus, everyone other than Garleans (and poor Thancred) are capable of subconsciously using magic to reinforce and empower their bodies, which is implied to be how non-Garleans pushed them to inhospitable territory and why they need magitek power armor to help close the gap. An axe in the real world can cut through wood; an axe in XIV's world can cut through stone, used properly.
P.S. The IVth having an Agrius sized vessel when it's supposed to be sorely strapped for resources irked me too; but it's XII fanservice, so they'll do it anyway.