A. The Garleans invade a country and mistreat the Majority group. Treat them less than human. The obsession of "purity" of the Garleans that were driven into a small country would make them the minority in every country but their own.
B. The Garleans are from a small country and are obsessed with purity. Their genocide depopulates their own Empire until there are barely enough to run basic services. They don't even bother protecting the people that have sworn loyalty to them either.
C. The Garleans aren't stripping resources from the Majority. They just destroy the resource output of the conquered until everyone is starving.
D. Who would be okay with this? And who would get rich under this system? Collaborators seemingly get no protection and the GDP of these occupied territories has to be abysmally small. And when was the last time saw a loyal Garlean actually do any work besides killing somebody or standing around waiting to be killed?
Perhaps you should read a history book. Empires are built on greed but the Garleans are not greedy. They see a farm, they torture and conscript the farmers until everyone is starving. They see a fishing village, they murder the fisherman. They see a quarry and beat and starve the workers. They see a saltmine and drive the people off so that the wildlife reclaims it. They see a paper mill and they destroy it and leave it in ruins. The Garleans do not profit from war, they aren't motivated by it and every action they take says otherwise. Their occupations makes the lands worthless and they must be spending fortunes building the towers, artillery pieces, mecha, shield generators and bases too. This "Empire" should have crumbled the moment the Garleans left Garlemald.
The sinews of war are infinite money, but the Garleans sidestep this because they are one dimensional villains. We learn rebuilding the Doman enclave that they stole all the good tools from the Domans. But we already know the Garleans and their magi-tech don't need medieval tools, they probably just chucked them in the ocean because that was the "cartoonishly evil" thing to do.