In every single case, the Scooby Doo gang discover a human is behind the mystery, not ghosts or mummies or whatever. Yet they start each new case believing the villain is a supernatural being. Every time. Not once do they say “maybe it’s an old man like last time”
@JoParkerBear maybe they only televised the ones where it turned out to be a human! Maybe all the ones with actual ghosts/monsters etc were deemed to scary for kids and never saw the light of day and there were way more of those, hence the assumption each time!