Because, yes, they are that stupid.
We know about dinosaurs because we had a large and enthusiastic media campaign to promote their discovery and study.
People who grew up in households that weren't exposed to Paleontology as a science, but got immersed in religious propaganda and mythology, came out with a totally different understanding of the world.
This has nothing to do with human intelligence. Although, the intrinsic need to stack rank every population into "Smarties" and "Dummies" is another instance of Calvinist propaganda (transmuted by Ayn Randian eugenic theories) tricking otherwise intelligent people into believing the craziest and most incoherent things.