Some quick "definitions" to help understand things. "Paranormal" simply means something unexplained or unexplainable. "Ghost" is a word that while it has a specific meaning, actually gets used to describe any sort of unexplainable occurence whether a visable one or one that is only felt or measured by various devices (such as an EMF detector or recording devices).
Typically, most people who experience paranormal things do so because they expect to. Whenever you go into a situation with preconceived notions about what to expect, you will usually not be disappointed. The same could be said for racism or any time when you look for certain things to be there whether they are or not.
Another opinion is that there is a spiritual "parallel" world where we sometimes see things, where we get glimpses of what is happening in the spiritual realm. They visions (for lack of a better term) is usually angels and demons, though some would say that it it may be the perverbial pergatory where spirits await judgment day.
You bring up a great point about how many ghosts there would be based on how many people have died. With the millions dying at any given time and the billions throughout history, why aren't there ghosts everywhere for everyone to see? Why is it still a debatable topic? I don't think that ghosts are what we think they are. I lean toward the angels and demons idea myself, but can conceive of the purgatory idea as well.
I don't know if this answer helps you or not, but it's what I think about it all. Take care and don't forget that you will see what you expect to see.