"Everyone else" could be wrong. There are too many commonalities between these victims stories, too much unique information delivered by the entity affecting them.
Schizophrenia practically did not exist prior to the 19th century, and modern psychology and psychiatry are based more on conjecture and 'reasoning' than they are on modern science. If you cannot find a physical cause for a mental disorder in neuroscience, I am more likely to believe that it is not a disorder caused by the human brain. Additionally, it is a disease whose onset is usually after the full development of the adult brain. Were there to be a neurological cause for the disease, it is highly unlikely that it would not be recognized during development.