When you make your living from performing, you need a balance, you also need to be spending the day time working on your set, not peddling your arse on the phone. Agents can do this for you and whilst there are good and bad ones out there, lazy ones as well as hard working ones, agents are the only route to having no other income.
The demise of the large folk club over the years has led acts to look elsewhere, and st least three agents I know accept that fill in gigs booked direct to folk clubs are a good marketing exercise so don't get precious about contracts.
The "money" is on the provincial theatre circuit and larger festivals, as well as the steady income from albums which are much much cheaper to produce these days, especially download. As noted above, agents are there for commercial not altruistic reasons. Most acts of a folk genre negotiate a contract that frees them for contributing to small folk venues, seeing them as publicity for larger bookings.
I have never been a full time professional but have been on the books of agents as a semi pro band member over the years and this is my take from what I have seen, as well as booking acts for folk clubs and theatres many years ago.