During the 3rd Umbral/Astral Eras (can't remember exactly which), there was an empire similar to the Garlean Empire called the Allag. They had taken over even more of the world then the Garleans and not only had mastery of Magiteck but of also magic in general as this was about the time that the schools of Black and White magics where formed. So even if they are doing a "better job" now of asserting control, it may just culminate in an arms race that sends the world back into an Umbral Era. Minfillia even states that people, beast tribe or no, turn to "gods" and "higher powers" when they are desperate and the Garlean Conquest simply fuels these feelings of uneasy and desperation.