There's a whole lot of science-related fail in fiction, with some fields of knowledge bearing the brunt worse than others. In the case of linguistics, the vast majority of people have no idea it exists, never mind the basics. Obviously, this includes writers. Indeed, the prevalence of this trope (and its relative lack of being noticed) can be attributed to this fact - most people recognize that when dealing with questions of physics, biology, chemistry, etc., they need to ask an expert (though many writers just don't care) - whereas with linguistics, most people don't even realize that there are experts to be asked, much less that their own knowledge is generally insufficient.