They don't. The only evidence that you have is that quote by Shameless O'Shamrock that you have used half a dozen times now despite the fact that it was refuted by Sailor Ron here and S O'S acknowledged that he was right.

Steady on there, Sycophantic Mollusc - I still stand by that list: all of that music I have heard in The Fleetwood Folk Club, not necessarily on the same night I grant, but all of it I accept as being Folk Music according the reasons given in the 1954 and All That thread - i.e. Folk as Flotsam, determined by context rather than content, and the nebulousness of the 1954 Definition which might as well be saying All music is Folk Music - I ain't never heard no horse sing a song. Furthermore, in my ongoing experience of what happens in Folk Clubs I'd say that list is pretty conservative.