Most people haven’t and there is a surprisingly simple reason; Einstein’s Theory of Special Relativity
and Time Dilation
. This isn't half baked, woolly hatted mumbo jumbo, it's hard core science. Put horribly crudely, a ghost is a very low mass/energy object for whom time is passing massively faster than it is for us.
It is generally reckoned that something that is in vision for less than 1/25th of a second
cannot be seen, from a ghost’s perspective, our 1/25th of a second is a few hours. Only by standing unmoving for hours at a time (in the ghost's Time Frame), can a ghost be visible to us. Doing this requires a great deal of motivation on the ghost’s part, this motivation is usually a desire to contact someone either to reassure them or terrorise them.