According to your logic("Satan's great weapon is that he doesn't exist")he would avoid all corporeal "contact"with the human race, i.e., possession, demonic influences, etc. It's true that he does not want us to know he exists, And not all haunted houses are demonically caused, but just because Satan has a superior intelligence does not mean that he is not also stupid. He is ruled by the passion of "pure" hatred and can't resist getting "involved" with God's, after the angels, highest creation, mankind. His superior spiritual intelligence, like his spiritual power, have been curtailed by Almighty God. You could say he can't resist the temptation. However, his intelligence is still vastly superior to ours except for the saints.