One difference between poetry and lyrics is that lyrics sort of fade into the background. They fade on the page and live on the stage when set to music.
Source: On Broadway musicals; in NY Times, 28 Mar 1987.
-- Stephen Sondheim, (Mar 22 1930-____), US composer, lyricist; His brilliance in matching words and music in dramatic situations broke new ground for Broadway musical theatre in West Side Story.