That is a feature. The game doesn't care if the ghost is played or not. A ghost is a ghost. Thus excluded from most sim_filters (especially from service sim roles).
Thus you won't see them out often unless a ghost is required (near their grave, Spooky lot challenge, when a holiday tradition makes ghosts wander in the streets, bar ghost nights, séance tables, haunting maintenance events etc.).
This has been so since ghosts were added to the game 1 month after the game was released and this hasn't changed since then. So yeah, doubt it'll change.
