Plumbing can be hard work. Being on your back / knees for hours at a time day after will end up getting to you. Also, it sounds like your doing some of the more reasonably easy plumbing. Things are a bit more complex when you have to do things like figure out how to maximize water pressure after N different horrendous bend angles, etc. If you think of it in terms of software engineering, the plumbing you've probably done is probably through intermediate software work. You've set up some CRUD apps at this point (in the plumbing world). There's a reason plumbing is a trade and a craft...it can get very complex and the time they're charging to fix a trivial flush valve on a toilet could just as easily be spent on a commercial plumbing gig with some corporate client who is paying the plumber to set up every toilet in a small hotel (for example).