@OsoJr I mean... wouldn't you be too, if you understood you were a sea slug? Though, the reasons for being in denial can be for varying reasons. For some, it is for the sake of their fragile sanity, considering they may not have been IN those forms once upon a time. One must remember that the world has to have been restarted multiple times before because of the records made by the SCP in Yellowstone that had the capability to restart humanity. Hell, a fan theory is that all SCPs are surviving beings from Earth cycles that have come and gone for one reason or another.
Others may legitimately not understand they are not what they think they are.