Ghosts are not the souls of the dead. Once you’re dead your soul leaves the earth plane. Ghosts are beings from another dimension, yet they see and understand what we’re doing in our dimension. We live in a ‘parallel-universe’ with them. In other words- they are not coming and we are not going. Our worlds overlap each other. They are simultaneously doing things in the exact same space that we occupy and at the same time too. Their habitual tendencies anchored them to this earth plane of existence where they share a similar frequency. But because we are at a different wavelength from them we do not disturb each other and normally they are invisible to us. Usually only a person with a high level of spiritual attainment can see them.
Sometimes a great magician or ‘medium,’ (person calling on the spirit), can manipulate an audiences’ frequency to see ghosts, like at a séance, but it requires a lot of energy from the medium. In Buddhist texts, many times, Buddha displayed very vivid scenes of pure realms before certain crowds of disciples.