I wonder whether the 'family' set up has something to do with it. When we were kids we would sing with mam, whether it was nursery rhymes, or songs she knew from 'community song' books, or the radio, etc, 'folk songs' learned in schools thanks to Sharp. Maybe if people sang in families and with kids then group singing was more common, but those from whom collectors collected songs were often past child-rearing stages of life (with a few exceptions of families being found).