Why? Because a Lyric, by itself, is NOT a Song. It is merely Verse.
A Song, at the very minimum, is defined by the Lyrics, the Melody, and the Chording, which is all competent musicians need to work out a viable arrangement and is usually embodied in a "Lead Sheet" which is commonly used in popular music to capture the essential elements of song without specifying how the song should be arranged or performed, (Although, practically speaking, the Time Signature and the Tempo should be indicated as well).
As Nolita and I discussed in a previous post, such is difficult to reproduce properly on a Forum like this, as Chords are formatted improperly and there is no convenient method for displaying, (or even indicating) Musical Notation, so the only workable alternative, if one is actually trying to present a SONG is to provide a link to the Music at another location, (Whether the Lyric is posted "here" or not).
Hope that answers your question.
Jim Hoye, (JRH)