Because there's many ghosts already, and the 2000s ghosts can't afford rent. This is why most places are NOT haunted. They are officially owned by few older ghosts, but they just rent them out, and the rent is too high to be afforded by most younger ghosts.
And those few that can afford it are too busy for any actual haunting.
Such is life death.
There have been several short windows in time where the soul pipeline was blocked because of [REDACTED], this leads to a buildup of departing souls and the most [REDACTED] of them end up getting dropped from the backlog database and, bam. Ghosts.
Just never ask them what they saw when they tried to [REDACTED] the [REDACTED]!
Ghost don't haunt places at 3am if there are cats in the vicinity - scheduling issues, cats have dibs that ghosts are bound to honour (they just move their haunting at midnight usually ... that is if there are no witches around which too have them outdibed, poor spectres have to haunt at the awkward 1:30am timeslot if they want to get any work done).