Because nobody wants ghosts in their apartment to begin with, but if anything could make that worse, it's probably a ghost crawling into your apartment covered with an entire apartment building's worth of sewage.
Thirteenth Floor (And Floors that End in Four in China)
"Wait a minute," you might say. "The number 13 is not a ghost." You are clearly someone who has never had to wake up to the number 13 rattling its chains in the next room or causing your child to say "REDRUM" over and over again. Anyway, unlucky numbers are specific concepts that people believe will do horrible things to them in a magical way, so close enough. If you're insistent on ghost purity, read ahead to the next entry.
Most people have heard about some Western buildings not having a 13th floor due to superstition about the number 13. The practice has been dying out, but some newer buildings still do it, like One Canada Square, the tallest building in Britain (until the Shard London Bridge is completed).
Doesn't really have much else going for it, does it?
One Canada Place gets around the problem by having no "usable space" on the 13th floor, and no button for it on the elevator. Other buildings just rename the 13th floor as "14," exposing the unsuspecting residents of the new "14th floor" to all kinds of magical catastrophes, unless the number 13 spirit is as stupid as a Chinese ghost and just wanders away disappointed after not finding its designated floor.