Once purchased zombies and ghosts are intended to be fairly rare especially compared to witches and bats. You never know when a zombie will pop up from behind a grave or over a hill but you can be sure it wont happen too often. Ghosts are very difficult to spot as they like to try and hide behind trees and buildings. Often you will only catch a glimpse of them as they hurry from one hiding spot to another.
The Grim Reaper is the rarest of all! Not only that, but by his nature he blends in very well with the background. When you do see him he will be as still as the grave in an effort to escape your notice.
If you are are just dying to see these rare creatures then you can always go into the settings and increase the monster frequency.