Two replies to that.
First, there's no coherent concept of what a real demon is ─ if we found a real candidate, we couldn't tell whether it was a supernatural being or not, a demon or not. This is the problem that calling it a 'spirit' (as in 'evil spirit') seeks to evade; but there's no way to distinguish a spirit, in this sense, from a fictitious being, something entirely imaginary, so that gets us nowhere.
Mental illness is real, and accounts for so much of 'demonic possession' as is not fraud &c. It's the subject of evidence-based research which continues to bring better understanding and better results. Like just about everything about humans, it remains a work in progress.
Second, if we shut our eyes to the first point, then Satan, and demons, as well as mental illnesses, can only exist because God wants them to exist. When the universe is ruled by an omnipotent, omniscient being, there's no other possible source, no other place for the buck to stop.