Of COURSE their songs are used in folk circles. Why not?
The main limiting factor is that a number of them are rather hard to sing without a piano. They take something from patter and music hall songs, so you have to be able to deliver words and tune clearly and quickly.
And it's sometimes hard to grasp the multiple key changes without adequate accompaniment.
I suspect a number of people who don't know that Tom Lehrer wrote "The Irish Ballad" (commonly called Rickety Tickety Tin), sing it at folk gatherings.
Ditto the "Hippopotamus Song".
Blessings,
Barbara