If everyone on this forum got together and made a list of everything physical (or mental) they have ever been concerned about, removed all the duplicates and further thinned the list down to only those things we sought medical advice on, I doubt there is any one single Doctor in existence who could diagnose everything. To train someone to do that would take more years than possible.
They are General Practitioners and like a plumber they need to go through a process to diagnose the problem. Even plumbers will, on occasions, scratch their **** and say "I have no idea what's wrong" and just replace the area the problem is in. Through experience they learn and have a higher diagnosis rate but will never be able to catch everything.
As an example, last year my 15yr old, fit and healthy, daughter was rushed to hospital with what, the paramedics were convinced, was a stroke. After all the scans and tests the consultant could not say that it was a stroke. She could not say for certain what it was and why it happened. It was referred to one of the best neuro's in the country and, again, no diagnosis.
Before dismissing GP's as a joke when they are uncertain remember that they, on a daily basis, catch things that save lives or prevent suffering and like the rest of us are human.