Why are Alolan Marowack, Sensu Oricorio,and Decidueye ghost types? They’re not dead!
Well, I don’t think being a Ghost-type necessarily means you are a literal ghost. Maybe half of them, tops, are said to be the spirits of the dead in legend and folklore (my rule here is “read the Pokédex because it’s really important, but for the love of Arceus, don’t trust it”), but most of the others have their type and abilities on the grounds of an affinity for death and the dead, or in some cases power over them. Pokémon like Dusknoir and Pumpkaboo, for instance, are known for leading spirits and people between the worlds of the living and the dead, while Chandelure and Jellicent are spiritual predators; I don’t think anyone ever claimed that any of them are the ghosts of dead people or Pokémon, but they can all interact with ghosts. Similarly, Decidueye is a sort of shaman with the ability to interact with souls as though they were physical things, and Alolan Marowak can summon the aid of spiritual powers through their ritual dances.