There has never been a claim - as far as I am aware, that 'nobody wrote them' - I would ask you to point out who has made such a statement but you don't do that sort of thing, do you?
The original authors of folk songs are unknown, but that is beside the point. It is the process of acceptance, oral transmission, adaptation and change that give songs their 'folk' identity, not their authorship.
Produce the name of 1 master craftsman - if you can't why can't you?
Jim Carroll