Where are they getting this millions of millions of troopers? Keeping them fed? Keeping them organized through the vastness of the galaxy? Aren't they then spending more resources than they are acquiring if they have to mantain a military operation on a PLANETARY SCALE?
That's the thing, they don't 'control' the planet in the sense that they hold jurisdiction and direct influence over every area of the planet. That's too unrealistic and expensive. As long as they get their resources, they're not attacked and no help is given to the rebels the Empire are quite happy to give control to crime lords and other pro-Imperial warlords. They don't control the planet with ground forces alone (please stop regurgitating that non-point), but use the threat of destruction to hold it. That's why they go overboard with super weapons, massive space ships and walkers. To evoke fear and the threat of utter destruction. Blowing up a planet is expensive and the Imperials are AWARE of that, but it also discourages rebellion. Not every citizen is going to be a insufferable nitpick (I'm joking) and think about how expensive destroying planets are, most are going to worry about keeping their families and themselves safe.
They're a fascist empire that uses the resources they gain from their planets to build starships, vehicles, guns and supplies so that they can continue their imperialism. Their entire society is built upon extreme militarism, and they continue expanding to stay relevant and escape stagnancy that plagued the Republic. They ensure they get their resources through space ships and ground troops that transport materials to safe locations where they can be transported. They do have ground forces, they use them as a symbolic sign of control, invading planets, protecting Imperial missions (like the transportation of goods and research centres) but not to safeguard every neighbourhood of a planet.
