Here's why:
* It refers to a piece of music: A song is a specific, identifiable piece of music with lyrics and melody.
* It can be the subject of a sentence: "The song is beautiful." (Song is the subject, the thing being described).
* It can be the object of a verb: "I love that song." (Song is the object, the thing being loved).
Therefore, because "song" names a specific thing (a piece of music), it functions as a noun in the English language.