You are 100% correct in that bike fitters are not doctors and I don't think that any of the certified pro fitters would claim to be one. They most likely do not know the in's-and-out's of joints and how to fix them. If you have a knee injury that is going untreated, no fitting in the world is going to help it get better. If you haven't consulted a doctor for it, my suggestion is that you do so, even if your family hasn't had much luck with them. I would try a sports medicine specialist. Once the knee issue is resolved, then a pro fit would be beneficial in helping to avoid a re-injury from riding.
With that said, I can tell you that I had a Retul fitting done just after I purchased my road bike and I will do it again if I purchase another road bike. It may sound like a lot of money, but when done by a certified fitter, it is the best money you will ever spend on anything related to cycling besides the bike.
Here is the link to the
Retul University. You can figure out for yourself what makes it so special. I don't think that Carmichael Training would be using it on Lance Armstrong and the Radio Shack team, and other pros, if it didn't work.