Contrary to what you've heard, God is not legally allowed to work on Earth without he permission and cooperation of man. So is the devil. They're both fighting for our attention and cooperation.

He's not in control of the world systems. Man was giving dominion and he seeded that mandate to the devil. So the devil is in control. let's stop saying "God is in control". He's only in control of the lives of those who have given the control to.

The legal entry to earth is through the womb of a woman. Not even God could defy that. So Jesus had to be born. And on the other side, "the children of rivers" and whatever also have the cooperation of man. So demons can enter and operate.

The system is already rigged for them because their master controls it. Which is why he had the legal right to ask Jesus to bow before him for a reward of earthly kingdoms. Which is why when Daniel prayed, the answer could be held by the spirit of the prince of Persia for 21 days

So God is looking for people to work through. He occasionally give glimpses, sneak peeks, trailers, messages and so on so people may believe. He's tasked those who believe in him to proclaim him and to use their sanctified works to win some for him.