It may also have something to do with the fact every Plumber I know makes really, really good money. The photographers I know, not so much. Also, you have to consider, nearly everyone in your area needs the services of a plumber at some point. Photography, not so much. The pool of people looking for photographers isn't nearly as large as the pool of people looking for plumbers. Thus, a lot of times, photographers can't afford to give their friends freebies, as they don't have enough of paying customers otherwise to sustain their business.