Why do some many people in The United States reject evolution?

I hear people say Evolution is only a "Theory" without even know what a theory in Science means. I do not hear people reject the Theory of Relativity or the THEORY of gravity. I guess they do not reject them because it is not in the bible. As an atheist i absolutely hate when people say dumb things like that. I mean it is okay to QUESTION evolution. But to deny it is another thing. They try to teach Creationism in school, when it isn't even science. I mean are some of us Americans just stupid? Is this why the world thinks Americans are Stupid?