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With the exception of special cases like Yamask, most Ghost-type pokemon aren't "ghosts" as we think of them; that is, most of them aren't spirits of deceased people/animals trapped in a state of purgatory or unrest. Most of them are biologic organisms in another dimension, existing in a state of flux between that dimension and ours. This is why they are intangible to normal/fighting attacks.
The first ghost pokemon in the series represent this idea. Ghastly is just a face, because it is only able to peer into our world. Haunter has begun pushing through, hence it is a head and hands. Gengar has pushed all the way through; the spikes on its back are just remnants of the dimensional change conforming around its body as it finally completes the travel. In their own dimension, all three species are just Gengar with differing dimension-hopping capabilities, and they are all tangible. There, they are poison monotypes.
Other ghost pokemon have similar flux between dimensions, but as an all-or-nothing capability. Misdreavus evolving into Mismagius does not represent breaking further into existence, because it has already achieved 100% existence into this reality. For this and other non-Ghastly-line ghost pokemon, evolution represents growth the same way as it does for non-ghost pokemon. The ghost typing is a vestigial component of being from another world.
In a sense, Ghost type pokemon are the first Ultra Beasts, though they are not recognized as such.
Yamask are an example of Ghost type organisms achieving symbiosis with human spirits, and Shuppet are an example of symbiosis with yokai/object-spirits. Spirits exist in the pokemon world but are neither necessary nor sufficient to be Ghost pokemon; the type's name is a misnomer due to human misunderstanding of their tendency to be found near places of spiritual import, such as cemeteries.
