Medications can help symptoms without knowing underlying cause. An example we are famiiiar with is tylenol for back pain. In the same way, doctors sometimes prescribe a medication like topiramate for migraine. It has some notorious side-effects which are worth it to some people, just to get even partial relief from the symptoms while not knowing what is causing the symptoms.

You are lucky. At least they have not started saying it is psychosomatic.

As for testing for nutrient deficiencies, You are right. They are not educated about that as well as they are about what the pharmaceutical reps teach them. So since nothing else is working for you, you might want to go to an out-of-network doctor not afraid to run tests. Insurance should still pay for the tests.