The reason is because they cannot see, measure or prove His existence. They forget that that which remains incomprehensible, mysterious, unfathomable, obscure and invisible to them is the God they ignorantly scorn. That is the true nature of God and he understands the predicament we face since we cannot understand Him, so He gave whoever is willing the light (Christ) to guide us in our mystery.
This Christ took up the body of a man called JESUS ( Joshua, Justus, Yeshua, etc.) to make atonement for our errors.