Just about any business proposition you can think of is more likely to make money than running a folk club. Even performers who are not doing it full time need money for gigs.
This week, on Thursday, I and my best friend will finish long working days, drive through the night from Cologne to Calais, get the ferry, and then drive to Cornwall. There are two small gigs there and then it is time for the drive home. We will get back most of our expenses. Don't ask a plumber to do that for you - he might refashion your nose! I have yet to meet the plumber who can learn my job in three years.
Most folk musicians - even those who are considered established professionals - make a fraction of what they would earn in any other profession. Even in the sixties/seventies heydays of folk clubs, that was the case. Folk clubs do their best to support folk artists, but it is a constant struggle for even the most successful. I am glad they exist at all and I don't begrudge them a penny.