the singular "melody" is appropriate when referring to the collection of notes ... but we want to make the words of a song plural.
That's because the words ARE plural, Flatlander. We don't perceive a succession of notes as individual notes (unless, of course, we are the composer). But each word in a song has recognizable meaning. Hence we think of the words of a song in the plural and we refer to them as the "lyrics" of a song. This is quite logical and any convention which flies in the face of logic will sooner or later succumb to the weight of that logic, I submit.