Find a traditional tune, and you're likely to find four or five sets of traditional lyrics for it.
Then, or course, there's John Gay's "The Beggar's Opera", where ALL the original songs are set to traditional tunes. Gay did this so audiences could sing along.
"Rosin the Beau" is also "Men of the West" and "Lincoln and Liberty". "Streets of Laredo" is also "The Bard of Armagh". Using a pre-existing tune is a time honored folk tradition.