Thank you for bringing this up for discussion.
This issue deals with the lore and the game system so I would like to explain them separately.
Currently, each grand company does not have enough military force to cover the entire region. Using Ul’dah as an example, the Immortals’ military forces are not sufficient enough to patrol and defend Thanalan. Essentially they are allowing the Amalj’aa tribe and Garlean Empire to invade.
With limited military forces, defense strategies are planned accordingly and critical areas are prioritized for military placement. With this said, if there are any attacks outside of the cities, militias and adventurers may participate in the battle. (These are the random Garlean Empire invasion live events)
Also, something that players do not see are the ensuing battles between Grand Company forces and the Garlean Empire forces.
Why do Garlean forces spread out their forces instead of concentrating them? Players may just be able to find out by playing the content in patch 1.22. And the world in version 2.0...?! Oh wait, if I reveal this Yoshi-P will kill me, so I’d better keep it a secret.
In patch 1.22 players will be able to fight enemies together with NPCs in “Hamlet Defense,” not alongside Grand Company forces but with the militia.
Also, in 2.0, not only will players fight enemy NPCs with ally NPCs, players will be able to convert enemy NPCs into allies by fulfilling certain requirements. Many interesting scenarios are in development!