I think ghosts, if anything, might just be people who are extremely sensitive to changes in the natural electromagnetic field seeing things. Such sensitivities have been known to cause visual or aural hallucinations, and many “haunted” locations do have uncommon magnetic field anomalies. Similar to how sightings of various entities more than likely stem from hallucinations caused by sleep paralysis.

There is also just paradolia in general; seeing faces in random noise, or hearing voices in random noise. This is basically what EVPs and “ghosts” in photographs are.