I have 10 reasons:
1. Ectoplasm is fictional. Nothing is made of it.
2. There is no evidence for the soul.
3. There's approximately 107 billion dead people. We should be
swamped by them if they all became ghosts.
4. Assuming you only become a ghost with unfinished business, that's at least a quarter of them. And what is encompassed by 'unfinished business'? I'd love to finish a video game. Do I get to come back as a ghost and finish it?
5. Ghostly animals have been reported. Assuming any animal can become a ghost, we should be swamped.
6. Ghosts are allergic to large crowds. No large group of people have seen the same ghost in the same instance. No ghost has ever shown up at the olympics or at the world cup.
7. Infrasound. Infrasound is sound too low for the human ear to process, but it still detects it. This causes it to become confused as to what is causing it and can cause auditory and/or visual hallucinations.
8. Sleep paralysis. During sleep the motor cortex of the brain is shut down to prevent you acting out your dreams. In some cases, upon waking up, the cortex remains deactivated. You are unable to breath properly and will often feel as though there is someone else in the room, but cannot move your head to see them.
9. The laws of physics. A ghost must be either affected or unaffected by the laws of physics. If it is affected, then it is affected by the Pauli Exclusion Principle, and is tangible; I could throw a brick at it. It also cannot move through walls. If it is unaffected, then it is unaffected by the Pauli Exclusion Principle and is also unaffected by gravity. With the Earth moving at approximately 60km/s they should be ripped away from the Earth at high speed.
10. Suggestion. Ghosts are commonly seen in haunted locations. Mainly because upon being told it is haunted, the power of suggestion takes hold and every breeze sounds like a voice, every noise like footsteps.
There's ten reasons that ghosts don't exist.