Why can't Jem accept scouts opinion that there is just one kind of folks--folks

Jem can't accept this because he believes there are four types of folks;

"There’s four kinds of folks in the world. There’s the ordinary kind like us and the neighbors, there’s the kind like the Cunninghams out in the woods, the kind like the Ewells down at the dump, and the Negroes."

You have to remember that there was a point he believed the same way as his sister, but growing up and experiencing life have clouded his view of humanity.