Every empire since the year dot captured land, with peasants farming it, who could be taxed, to pay the soldiers, to capture land... that was the business model. A little profit was left over to pay for cities, palaces, pyramids, etc.
This flow of money from country to towns was reversed by the industrial revolution. Within almost every country now, the cities are taxed to pay the rural voter base (or to pay off the rural might-be-revolutionaries, in places with less voting).
Not the only thing going on, of course -- changes in weapons matter to the balance, as mentioned. So does the rise of literacy & nationalism, which made the idea of rulers who are too distant (by ethnicity, language, or religion) from the population base (as they were in empires, almost by definition) into a rallying point. And thus raised the cost of holding land.
The tools of modern warfare are too widespread and easy to adopt. You can’t do it anymore, the old school empire is just as dead as old school fortresses. Technology has broken it.
I have friends who served in Afghanistan. I’m not unfamiliar with how powerful the US military is. But even that power is too weak.
America’s approach is way more resilient and the control exerted is much easier to retain.