As some have pointed out it would likely work against Satan’s cause if people knew that Satan existed, assuming that Satan’s cause is to get people to reject and disobey God. If they knew Satan existed, then more would know that God existed.
And as I pointed out, all God would have to do to wreck Satan's plan of being hidden is to show himself. It's not a hiding competition.
The absurd Christian assumption is that knowledge of God suddenly makes things easier, like it's primarily a test of faith. The winners are people who can deduce the existence of God, with the least evidence. God doesn't like scientists! God will punish 500 years-worth of mankind, because we realised that God was not holding up the moon. Prior to science understanding gravity, the rotation of the moon was perfect proof of God, so God was revealing himself, and apparently cheating by not cheating. Currently, we cannot explain consciousness and the Big Bang, so God is still revealing himself to us in things we cannot understand. God of Gaps revelation.
Why doesn't Satan have this advantage? Why did we not look at the moon and assume that Satan was making it rotate? Is Satan not powerful enough to rotate a paltry moon? I think it's time we liberated Satan and ascribed all mysterious things to him. Obviously, mysterious things are mystifying, and only Satan would mystify us.
What if God showed up, and said "I'm here! But I'm not going to tell you which religion is true. Maybe none." We would learn nothing from that appearance. It would take a lot of hints and good shows, for us to even guess what God wanted from us, or whether we were even supposed to worship him at all. It would cause a lot of conflict, as extremists vied for the true religion.
We might ask God a question : Do you want us to love each other?
God would reply: Yes, because if you don't, you will kill each other and wipe each other out.
We would reply: So, we were right! You do want us to love each other.
God would reply: That's not a religion, it's just a survival strategy. Chimps and rats have managed to survive. Bravo!
God or Satan could reveal himself an awful lot, without giving us an idea of what we were supposed to do. Satan could appear as the Indians depict Kali. We would be screwed. Everyone would conclude that the Indian religions were true, and flock to them, like idiots.
A plausible reason why God does not reveal himself is... wait for it.... He does not want to be worshiped, or to cause conflict by speculation.
Another plausible reason is that neither of them can reveal themselves, because our notions of God and Satan are so childish and stupid that it would be as pointless as revealing the source code of Linux to a caveman. We want to be congratulated for doing what rats do.