Many countries have gone to war with one another long ago in the real world. Bad blood still exists to an extent in the present day, so it's strange to pretend as if the passage of time suddenly renders everything obsolete. In some cases, it is the actual culture of the invading nation that people oppose - and Ala Mhigo's culture has not changed much at all since the days of its attempted invasion of Eorzea. Ala Mhigo still values violence to a significant degree. It still worships a deity associated with destruction. Many of its people act under the belief that the world owes them something - as seen with the horrific behaviour of many of the refugees.

I also wouldn't really class Hydaelyn as a 'medieval setting'. There's certainly a lot of risks that lower life expectancy but there's many elderly NPC's to be seen throughout the game world...including combatants.