I've explained this to people, only to have them say, "A good mechanic won't have those problems. It's the incompetent people that are the problem!" Competence is a big issue in the auto repair field, and in the absence of standards and certification, a qualified tech is hard to find. But the thing is, the most competent mechanic in the world still can't see inside your car without taking it apart. None of us have x-ray vision. Surprises and complications happen to the best of us. To say otherwise is to deny reality.
That is particularly true for a shop like ours, where we specialize in difficult jobs, and may of the cars we work on are referrals from other shops. The "easy fixes" have already been tried, without success. How does one estimate what it will take to go forward to the end? Many times, we can't. We can only price each step as we take it.
Often we take step 1 without even knowing what step 3 might be, or if there will be a step 4 or 5. We have to be flexible and figure a path as we go. Medicine and other complex diagnostic processes work in a similar manner. You do a test, and that leads to another test, and a treatment, and eventually - you hope - to a fix or cure.