Why do so many people disbelieve in ghosts and paranormal activity?
For many people belief is based on evidence. While some people may claim to have had personal experiences of ghosts and other paranormal phenomena, other people have not had such experiences. Without any personal experience, there is no compelling reason to believe that such phenomena actually exist.
Even more convincing is that so far, when paranormal claims have been subjected to empirical investigation by neutral scientists, no evidence has been found to prove the paranormal claims. On the other hand, there are often scientific explanations of why people believe in such phenomena which do not include presuming that the phenomena actually happened. For example, many people have very vivid dreams or are susceptible to the power of suggestion.
In some cases, such as psychics who charge money for their putative paranormal expertise, claims of the paranormal edge over into the fraudulent, with psychics and mediums often using sophisticated psychological tactics to essentially defraud the ignorant.