How has the empire not just obliterated the rest of the world?

[Gobio]

The 'power leveling' in FF14 is always dumb, since ARR.

In ARR, your primal destroying warrior of light must flee the city, and supposedly all of your friends must sacrifice themselves so a primal destroying god like entity like you can escape

In Heavensward, some human lancers can killed a dragon that was supposedly so powerful that its eyes that can partially fuel a primal that can destroy a asican, and then also fuel the most powerful primal that matches ultimate weapon.

So, when the empire attack ishgard or something, just send those dragon killing lancers to poke a few times, and the empire will be wiped out.

[TurtlesAWD]

The Garlean empire seems pretty analogous to the british empire, and while the british empire did indeed trample over the lives of millions using more advanced weaponry there's a lot to consider about, say, how the use of violence does not make one immune to retaliatory violence, the changes of public opinion towards imperialism on the homefront, the ways conquered/subjugated people can reignite a rebellion in areas thought to be under control of the empire, and the ways mid-level and regional incompetence/mismanagement (if not direct infighting within the conquering culture itself) can lead to control of territories being lost or conceded, often in a snowball effect.

Any sufficiently advanced military could probably render the planet uninhabitable but so far to date even the most brain poisoned imperialists in the real world have maintained a base-level understanding of how that would negatively impact their own self-preservation. I guess we'll see if that holds true of america's billionaires though, depending on if they think they can escape to mars with their profit margins intact. To that end it's entirely believable the garleans would not have destroyed the world as, A) it's not their goal and B) those who have had similar goals of conquest in the real world ultimately failed and we're able to experience that in reality.