>>47222623In Japan spirits and ghosts are not automatically dark and evil, more mystical and otherworldly. They're usually neutral and mischievous spirits. There's some dickbag ghosts don't get me wrong but there's many more that are benevolent and kind, or just a little odd.
This is the reason why Lunala is Psychic/Ghost. It's a Benevolent Moon Spirit. "Dark" is a better translation for overseas than calling the type literally just "Evil" but Ghosts have the moves that are related to shadows and darkness while Dark Types have moves relating more to underhandedness and brutality.
So Ghosts are weak to Dark because they're unable to fight back against something that evil. Think of it like a lost soul trying to and failing to reclaim itself from a demon. The Demon has the upper hand and will always prevail in torturing the ghost; it would need outside intervention like an Angel to help.