The mind. The hollow void of your skull and electricity that keeps you alive, based on ever changing but firm instincts. The very mass that is you. Your consciousness, all in one small, fragile pack of tissue. But as we know, electricity can be manipulated. You, can be manipulated. Everything you know, say, think, and do is all electricity. Like an overgrowing photography collection, everything passes through the brain to be confirmed, and then remembered. As an old saying goes, you forget a thousand things every day.
Brain from xray So, if the brain was such a good firewall, why would you still see shapes in the middle of the night? Eerie noises haunting you, only to be confirmed as something else? Well, it is simple. Why are humans afraid of snakes, storms, and war? We learned out lesson. We found that these things kill us, and we should stay far away from any source of these events. So as for these shapes, we learned our lesson. We found that these things can also kill us, just as the previous threats.
What phobia is pulling the threads, telling your brain to be wary of something you are told you are making up? All the studies, all the facts, and yet you still fear your midnight hallucinations. It is less simple than much would like to prove, but can be summed up with one word. Coexistence.
Just because humans forgot these beings exist, does not mean they have not slaughtered us in the past, and they certainly have not gone away. These stealthy beasts have taken kids, who were declared missing. They look like anything they can copy, that being why human shapes and beast like shapes both appear.
You forget a thousand things every day. Don't forget again.