Being able to see the future does not mean they are smart. Just because they can see it, does not mean they understand it.
Being able to posses people does not mean you are smart either.
All those traits you listed for Psychics and Ghosts as reasons to why they are smart are all false. Just because they have the ability to do those things does not mean they are smart. If I can read minds, it would not mean I am smart, even if I get 100% in all my tests. All it means is I am reading the minds of all the smart kids in class.