You must also consider the nature of our business. Plumbers spend a lot of time traveling around town from job to job. As soon as I get in my car to come to do a quote or a job, I use time and money just to get to their front door. Travel limits how much time plumbers can actually spend at someone's home or business doing work.
Doing this work at your facility is the only time we have a chance to earn money. Another factor that adds to the cost are the materials used in a simple plumbing job. Most people can unclog a drain on their own, so the problems plumbers face often involve repairs and other restoration services. The parts needed to complete normal plumbing work aren't cheap, and the cost of these things is often added to the total bill.
There are two main ways a plumber sets their rates. You can pay them by the hour or with a fixed rate. And, in reality, they can charge a lot because people will pay for it. It's true that they have to pay for certification, licenses, insurance, and equipment (there's no doubt that there are costs of doing business to cover), as well as their time and a premium for their experience (even if a plumber's experience sounds dubious).