why do people believe in ghost?

Because the human mind is at it's core, highly irrational. This is exhaustively studied, and the scientific method is designed to counteract individual (unavoidable) irrationality and bias.

The effects of this irrationality are everywhere, and not just prevalent in ghost-belief. People gamble (guaranteed loss), but don't invest in the stock market (proper investment means guaranteed gain in the long run). People smoke (actively harm their health), and don't exercise (actively do the one things that's most effective at improving and preserving health). Expensive placebos (fake drugs) work better than cheap ones. (The same is true for real drug -- even when the medication is identical, people report more benefit when it cost them more.

Probably because it's universal to all cultures in a way and so many people report seeing them that to many it may be hard to believe there's "nothing"

To me a ghost is not really that crazy or far fetched. There's way crazier **** than that which already exists so I would't be the least bit surprised if they someone proved ghosts were real or had some concrete proof.

Also I think the human mind is pretty crazy. Our minds in a way are almost supernatural themselves and arguably they might be. We can't even explain or find out what consciousness is, we can't found our own minds let alone ghosts.

People believe in spirits, ghosts, etc. because they think reality isn't interesting enough for them.

Which particular ghost are you referring to?

Idiocy.

I do because HAVE seen one......