Yeah, I don’t think “mentally ill people” only exist in specific places or that specific areas are “full” of them. I mean, I know for sure at least one deranged person lives in your area.
Also I highly doubt you truly worked in retail if you never experienced a customer like this. If you did, you must have been lucky and in an extremely high end store.
People are the same everywhere in the world, no better or worse than anywhere else. Which is kinda great IMO.