Butter is a key component of a quintessential American sandwich - the grilled cheese. Also, bread and butter is a midwestern dinner table staple, and buttered crackers are things I snack on, I put them out for dinner parties. Though, older people use margarine for "health reasons".
If I want tanginess on a sandwich, I reach for mustard.
I suspect most Americans use butter for the salinity rather than the flavor.
I further suspect that people butter their grilled cheese not for flavor, but for crisping the bread and building a barrier against the grease released by the melting cheese.
To YOU it is (and I agree with you). But people make it with oil or mayo as the fat. I doubt most people think of butter as a required ingredient.