The organisers either have to have someone available nearly full time to handle the bookings, a 'willing volunteer', who are becoming less available these days due to many pressures, including financial, so that many of these would prefer to be trying to earn some income; a fully paid 'ex-volunteer', which would cost probably more than getting a professional business to do it, or someone who gets a 'commission' for all the faffing around needed; or you pay a fee to a 'professional organisation'.

If such an organisation is handling many events, they can amortise their overheads over all of them, irrespective of how many or few booking are coming in at any time. If it is a professional business, only a fool would not be trying to make a profit.