A couple of points: Robert Burns wrote his stuff mostly to trad tunes. He was a lover of Scottish melody, and my understanding is that he put words to the tunes to help preserve them. (Although many have had other tunes written for them since...). In many cases, he re-wrote existing songs. So his songs were written as songs, not as poetry.
And I'm not too sure about GUEST's comment about poetry's 'bastard sibling, storytelling'.
I could be talking out of my arse (I'm in the right place, then....), but I suspect that storytelling is much older - only exceeded in antiquity by the Bare-faced LIe. In which case, it may be that poetry grew from storytelling.
Discuss NICELY.

John Kelly