Discrepancy in lore (spoilers)

[Brightamethyst]

The reason why Garlean empire didnt conquered Eorzea is basically plot armor

True, but Eorzea also has other factors that make it a harder nut to crack than someplace like Doma or Dalmasca. Even Ala Mhigo probably only fell because it was already unstable (and alone, after the Autumn War) when Garlemald invaded. You've got dragons, primals, a city state who's entire army is specifically trained for anti-air tactics, and aether so dense it literally makes people sick until they get used to it (though the writers have been mostly ignoring that last one these days.) That, combined with the fact that the goal is conquest, not annihilation, makes a takeover much harder.