Since the notorious Rosenhan experiment experiment, the diagnosis of mental illness and neurological conditions has vastly improved, your complaint pertains to the 1973 not today. That experiment was one of the reasons why the DSM was developed, that aims to rigorously categorises the symptoms of psychological and neurological conditions.
Admittedly, the DSM still relies on a symptom check list not objective tests but there are exciting recent developments where fMRI, EEG and genetics are beginning to aid diagnosis rather then subjective judgement alone.
Mental illnesses will be routinely diagnosed with the aid of medical scanning eventually.
Duffy, F.H. & Als, H., 2012. A stable pattern of EEG spectral coherence distinguishes children with autism from neuro-typical controls - a large case control study. BMC Medicine, 10, 64.
Funai, A., Bharadwaj, H. & Grissom, W., 2009. Final Report: Improved Discrimination of Asperger Patients using fMRI and Machine Learning.
Nieuwenhuis, M., van Haren, N.E.M., Hulshoff Pol, H.E., Cahn, W., Kahn, R.S. & Schnack, H.G., 2012. Classification of schizophrenia patients and healthy controls from structural MRI scans in two large independent samples. NeuroImage, 61, 606–612.
Schnack, H.G., Nieuwenhuis, M., van Haren, N.E.M., Abramovic, L., Scheewe, T.W., Brouwer, R.M., Hulshoff Pol, H.E. & Kahn, R.S., 2014. Can structural MRI aid in clinical classification? A machine learning study in two independent samples of patients with schizophrenia, bipolar disorder and healthy subjects. NeuroImage, 84, 299