I think it is mainly because he is testing our faith because if he just appeared everyone would worship him but that is not true faith if you were a humanist before because christianity and other religions are about FAITH so we have a perfect reason for him not proving his own existence
I see proof in nature, not in humanity. And, can you prove God doesn't exist? No. So it comes down to a personal relationship with God. Your relationship is different than mine, etc. Divine Nature surely has fingerprints of a Divine Presence.