I think, the reason why people do not believe in rebirth and karma, why secular people do, is because they don't really understand what they are.
They only have a pop culture understanding, for example, that the universe is busy rewarding and punishing people, or that the salt, fat, water and amino acids in the brain can somehow experience their own activity.
karma and rebirth do not actually have to be accepted on mere say-so or on faith alone. they can be determined by logical deduction.
In referring to nagas, or ghosts or whatever, they cannot be determined by deduction.
Profile Picture: "The Foaming Monk"
The Chinese characters are Fo (buddha) and Ming (bright). The image is of a student of Buddhism, who, imagining himself to be a monk, and not understanding the true meaning of the words takes the sound of the words literally. Likewise, People on web forums sometime seem to be foaming at the mouth. Original painting by P.Volker /used by permission.