This is just my definition if I were trying to make some logic out of it. It may not be true but still, hear me out for giggles.
A ghost is an enigma or blue print of an energy that once lived. So if I want to find the ghost of a civil war soldier, I go to where they battled and lives were lost. I do not go looking for them at toys r us so of speak.
If a ghost is only concentrated on lets say Transylvania then I'll call bullchit. It just so happens that ghostly stories are universally sound. Every nations to every cultures have seen it in their own neck of the woods. It is this universal uniform that proves to me that there is something there.
Now to break it down further, if it seems like everyone should have a ghost once they die, this then is an untrue statement. Why is that the case? Don't ask me, I just don't know but die of a tragic accident then maybe, just maybe you may.
The way I try to explain it is like taking a nap and you wake up from it but not fully awaken. Your body is still trying to catch up with your brain, you still get that feeling of tiredness and confusion but instead of waken up fully, you are still at that confusion state. Some people die without even knowing that they die. Death comes so fast and without warning. If you don't know if you have died then what?