I appreciate the mathematical analysis of music, but it's important to note that Western music originated out of liturgical chanting (Organum) and folk music. Since the composers of both were generally writing to texts, they naturally placed musical phrase-endings (cadences) at the end of phrases. Therefore, music naturally followed our preconceived ideas of language. Furthermore, since musical understanding is primarily a learned phenomena (compare South-East Asian music with Western; both cultures appreciate their own music first but can learn the other's), it is natural that our learned conception of melody would continue in its textual beginnnings simply through continual, generational reinforcement of the format of melodic conception.