Because this has to do with western medicine as a discipline and practice, in which we over-objectivized health care and the doctor/patient relationship is heavily asymetric. This leads to the fact that in most cases, doctors do not really listen to their patients, let alone believe them when they don't manage to find objective cues of a illness.

They easily discard patients' subjective experience. Recent research is calling for a change in that aspect, and change is slowly happening.