Simply put, just because something shows up on an MRI doesn't mean that's the cause of the patient's symptoms. For example, many studies have been done now that compare MRIs of healthy, "normal" adults with MRIs of back pain patients. They found there's no big difference between the two groups.
The people who don't have any back pain have just as many age-related changes in the spine. They have just as many pinched nerves and bulging discs. Using MRIs as the only diagnostic tool isn't realistic. The doctor must combine patient history and clinical exam with results of X-rays, MRIs, CT scans or other imaging studies. Lab values are sometimes helpful too.
It's clear that we don't understand what causes back pain in one person and not another. In fact it's not even clear why one patient would be pain free one day and develop painful symptoms the next.
Until this can be sorted out, when it comes to back pain, doctors don't have one tool to use as their "gold standard".