Why do you have to believe in ghosts if you believe in god? why is there such a compulsion?

why can't you just believe in god and not in ghosts? is it considered heresy or some other sort of sin by not believing in ghosts and the devil? i know plenty of people who believe in god but not in ghosts, but people tell them that if they believe in god they should believe in ghosts too? since i don't believe in god, i don't need to believe in ghosts either