'but from the many illiterate men who worked the earth and sailed the seas.'

They may not have been illiterate. They just didn't live at a time where people registered themselves owners of intellectual property. To write something like say The Cutty Wren or Sir Patrick Spens argues considerable intellligence and education. Certainly a command of and familiarity with creative processes. This stuff didn't just spring from the dull earth.

Their work may simply not have have had a discernabble value to people who account themselves arbiters of what is an important folksong.

You can see the same sort of process at work even to this day.

They might be anon or trad, but only because some gang of self important arseholes whose job it was to chronicle the times couldn't see the value.