my theory is that ghost pokemon aren't actually dead. "ghost" is just a term given to them to describe their phantasmal abilities. plus ghosts are not supposed to be able to interact with the living realm but then ghost pokemon are affected by the attacks of other pokemon. a could hit a with a wing attack even though is supposedly in another dimension.

also, in the episodes where ash went to lavander town to catch a ghost pokemon, he and pikachu temporarily died. they were ghosts but they still looked the same. and in the game. the marowak in pokemon tower is said to be a ghost yet it doesn't look like a ghost pokemon.