Maybe Roud explains this. Given that so much of the standard British song repertoire comes from known authors, why isn't their authorship better acknowledged?

In Turkish bardic song (roughly comparable in its position to the Child ballad corpus) the great majority of the repertoire was composed by known authors who are invariably recognized - ok, it helps that they usually worked their own names into the last verse, but that could been dropped or munged if singers had wanted to. So everybody knows which songs go back to Yunus Emre (contemporary with Chaucer) or Pir Sultan Abdal (contemporary with William Dunbar) - and there are many songs by both of them which are still sung regularly. In Anglo-Scottish song, Anon has staked out copyright to everything before Burns and most of what came after.