Master of the freak show
I don't think that was his point so much as it's never really explained why there's an abandoned mansion in the middle of a forest, why it's full of ghost types, and who exactly the other ghosts are (the only one I can think of off the top of my head is the man in the dining room).Ghost-types aren't really your typical "late-game"-type pokémon. Sure, it took a while in R/B before you could actually catch Gastly and Misdreavus has never been on my team in Silver/Crystal for one obvious reason, but since then, they're easier to obtain. Heck, Gastly is right there in Silver for you to have before the very first gym in the Sprout Tower.
In GenIII, it's Sableye in the Dewford Cave, or you can evolve Nincada and have Shedinja. In GenIV, Drifloon is right there at the Valley Windworks on Friday and like you said, the Gastly-line in Azalea Forest. In GenV, Yamask is capturable before Burgh and in GenVI you can have Honedge before facing Grant.