Creatures
Those who subconsciously delude themselves do so for the sake of their fragile sanity. Those who know and understand at least some aspect of the Shadow tide maintain the secret to avoid ridicule or investigation, as well as to protect the public illusion of a sane, mundane world.
In addition, since Shadow creatures really don’t belong to this reality, they have a tendency to slowly fade away. Some of them disappear within 1d4 rounds of dying (being reduced to –10 hit points), and none remain in this reality for more than 1d4 hours before the tide of Shadow reclaims them.