In the beginning, Satan intended to rule mankind like God ruled the angels looking at the prestige that was conferred on him by the almighty. Satan failed and riled up 1/3 of the angels against God in deceit, questioning His supremacy and leadership over all creation, why can't someone rule too, what if there is a greater leader. What would be your response if an employee you hired manages to convince a third of your workers that you may not be the best to lead your own company?
A good response would be to sack this individual and possibly withhold some allowances to those that fell for his 'deceit'.

A problem this solution may create however Is that the third that were convinced would be lost forever, psychological, they will work at reduced output wondering forever if the lie was ever true and one feeling confident will resurface from time to time to challenge your authority.

God handled this the best way possible, allow Himself and Satan to rule a sect of his creation( mankind) then let the two leaderships be put to test. In the end those that fell for the deceit will see the error of their ways.

But how does God compete with his own creation, he is all powerful, Satan wouldn't even lift a finger before he loses, how can there be as it is a 'level playing field'?

God in His wisdom then gave His condition for rulership. "Whoever calls upon the name of the Lord shall be saved".

I often Hear people ask why God won't appear in the sky even would be convinced , well there you have it.