The difference between singers and instrumentalists is that, apart from those with a physical reason not to be able to do so, anyone can open their mouth and start singing; whereas an instrumentalist has had to buy or otherwise acquire an instrument and learn at least some basic skills before they can join in even a slow session. So the instrumentalists are more likely to be aware of their own level and limitations.

However, although they can, people so often don't just open their mouths and sing. Largely gone are the days of family and pub singalongs around the piano, and singing (or whistling) at work. Folk club singarounds at least try to replicate an environment where everyone that wants to can have a go at singing, either solo, or joining in the choruses. I doubt whether very many people would go along to a session called, for example, "For beginners: come and improve your singing technique", as opposed to a friendly and relaxed folk club singaround.