Why is so diffucult to get effective treatment for chronic pain, fibromyalgia, deg joint disease, neuropathy which is all documented?

Tricky conditions. Fibromyalgia because it remains somewhat controversial can rub some md's the wrong way. There are excellent non-narcotic remedies for pain and neuropathy - Gabapentin and lyrica (pregabalin). Fibromyalgia/pain responds to cymbalta. Same has been shown to have benefits in some trials. Ani-inflammatories like Ibuprofen can provide relief for arthritis. Physical therapy, acupuncture, stretching, moderate exercis.
Fear of Abuse. I believe what you mean is why are doctors reluctant to treat these conditions, right? The problem we have today is that scores of patients approch us for treatment with opioids (narcotics) whose real intention is to abuse or divert (sell) these medications. That makes us suspect of all patients, many of whom are legit. We have a number of tools that separate the legit from non-legit patients.
Difficult to treat. Unlike many other medical problems, these conditions are very difficult to treat and there is no one medication or procedure that will fix these problems. Unfortunately, medications like narcotics can make these problems worse over the long term despite giving short term relief. Narcotics are not always the answer for these conditions. The best treatment is a multidisciplinary approach.
No objective tests. All of the pain syndromes, like most psychiatric illnesses, are almost completely subjective, and therefore, very difficult to accurately diagnose. There are no objective blood tests, x rays, and the like. There is nothing objective to see or measure. Lastly, treatment is an individual trial and takes a lot of time. Best treatment is from a specialist.