Eorzea is helped by plot armor but there are viable reasons invasions have failed.

First, since the VIIth had the then-newly reborn Alliance dead to rights at Cartenau, it's expanded its arsenal, trained, and adapted to Imperial tactics. Originally the Alliance was just the 1.x city-states: Limsa Lominsa, Gridania, and Ul'dah. They managed to scrape out a win against the XIVth alone thanks to tactical utilization of its forces to the fullest and exploiting weaknesses in Imperial technology (and a teensy bit of help from Hydaelyn, but then again Gaius indirectly had help from Zodiark so it evens out). The next major clash with the Empire came in the Stormblood conflict, after adding Ishgard's considerable arsenal of anti-air weapons and tactics to its repertoire. Finally the hypothetical war with the Empire that played out in the original timeline had both Ala Mhigo and Doma in the mix; neither want for martial prowess.

The Empire's technology can only do so much. It was able to blitz and conquer a wide swathe of territory, but other nations have wised up to how to counter it's technology and strategies and begun to band together to resist (or outright defy) its iron fist.

Second, the Empire's wide swathe of provincial territories is itself a handicap: because it generally mistreats conquered peoples, the Empire needs to keep its legions garrisoned in the provinces to discourage and quash rebellions. That means no matter how powerful its military might be it can't commit all its forces to an offensive without taking a huge risk. If it could I have little doubt Eorzea would fall like dominoes, but the Empire's provincial territories and how they're (generally) managed mean it can't do an all-out offense; meanwhile the Eorzeans are fighting just on one front (Ghimlyt) with all their forces (minus primal-slaying special ops, presumably).

We've never seen an Aurora-class cruiser deployed by the Empire; the only one we've seen at all is Gaius standing on the bridge of his during the End of an Era FMV. Considering each legion's legatus has one as a flagship, I assume they keep them in reserve as symbols of power and authority. The Agrius-class has an abysmal combat record. Nobody knows where the kupo the IVth got that airship or what it can do (but it must needs be there for XII reference). The biggest other airships we've seen are smaller escort cruisers... which can be shot down with artillery fire, as we do during Castrum Meridianum.

For all its power, technology, and advantages, the Garlean Empire is not invincible.