Why can't doctors put toghether my symptoms and come up with something?

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I feel like they are so inept. I have many symptoms of auto immune disorders... but the bloodwork always come up negative. I have uveitis, pnuemonitis, muscle pain, and various other issues such as hyperpigmentation. None of the many doctors I have been to can identify any cause. Why do they try to treat one symptom at a time and not come up with a cumulative cause?