To continue to sing a song doesn't make it traditional - it's far more complicated than that, but that seems to be what Roud's book is based on.

Perhaps your concept of "traditional" or "folk" is simply irrelevant to what Roud is investigating? He's looking at the songs people actually sang, and the social praxis of singing.

If you want there to be a book about the songs you classifiy as "folk", perhaps you'd better write it yourself rather than complain when other people spend years of their lives and a great deal of original thought writing about something entirely different, and which a lot of us out here want to know about.