To put this another way, two hypothetical languages developed in complete isolation from each other would be completely different. Two musical traditions developed in complete isolation would be different, but not completely different. There is still an underlying theory that unifies them to some extent. This is why music theory exists as a useful topic of study, even if it is not sufficient for writing aesthetically pleasing music. Whereas I know of no equivalent for linguistics: linguistics is truly a descriptive field, it can only describe what exists, not predict what properties a new language can/should have.