... or so they would have you believe. But if language is so immensely complex, so dreadfully difficult that no other animal has it like we humans have it, how is it that we do not go blind and deaf from neural overload whenever we use it?
Or is the complexity of language a mere artefact of the linguistic theories which try to account for it? Think of pre-Newtonian astronomy, with its deferents, epicycles piled upon epicycles, and other horrors of hair-raising intricacy. The geocentric model was wrong, whence the complexity. An inept model endlessly generates problems with itself, as it can never quite account for the reality which it tries to describe. Problems which cry for solutions, which can only be inept again. As the model, the theory, grows in complexity, so it does in obscurity. Just understanding it requires a life-time of study. Its proponents and proselytes are then secure. If you point out the ineptitude of it, they naturally counter you with a flood of mumbo-jumbo which would take you a life-time of study to refute.
Read the fable of Borne and the Wheel in the Kingdom of Pork, and Metalleus's tutorial on how to make your own linguistic theory and hope to attract some Phunding.