I think the problem is with the "who" in the presumption of "folk" music. Most people here seem to be defining "folk" as either British Isles or British Isles descended traditional music. The opening post to this thread laments that Britain (England largely, as that is where most of the British posters reside, it seems) has "folk" but the US doesn't.

Polka bands are folk. Mariachi bands are folk. Singer-songwriters are folk. Shape note singers are folk. Bluegrass and new grass are folk. A lot of hip hop and rap music is folk which never makes it into the commercial mainstream.

That is the biggest problem I have with this forum. The narrowness of the definition of folk.