It is very hard to approach the subject of ghosts, especially since people can only claim they have seen them or they have very little evidence.  Most people associate ghosts with people who have died in a terrible way or people who have died and are too stubborn to move on.  Ghosts first came from religions promising an afterlife to where people go when they die.  Ghosts are associated with culture, religion, and society, which made them very popular in the 21st century.  They were used in movies and books for the soul purpose of entertainment.  I believe the reason they could use ghosts for entertainment is because ghosts are not officially part of any religion.  It is impossible to know what happens after someone dies, therefore it is all rumor and beliefs.
I for one, do believe in ghosts and an afterlife.  Why not, people loved to be scared and having ghosts around makes life a little more scary and dangerous.  Having ghosts around would make people's adventurous side surface.
My sources are www.truthaboutdeath.com/blog/id/1586/are-ghosts-real.aspx and my opinion.