At least part of the reason is that the fundamental issue of biomedical psychiatry is more profound than whether phrenology was true. Phrenology was the first doctrine to suggest that cortical functions could be localised. Even though the specific hypothesis that feeling the shape of the skull could detect the relative size of the underlying organs of mental faculties was shown to be false, people have continued to think that brain functions must be localised in the brain at least to some extent. Hence the vast number of false claims that mental illness is due to abnormalities in particular areas of the brain.
There have always been theories of brain function. However, it was not really until the nineteenth century that science seriously considered that the cerebral cortex might be divided into distinct parts responsible for different functions. The cerebral hemispheres consist of a system of sensory and motor centres. But the subjective aspects of brain functions are more obviously part of psychology than neurology. As far as consciousness in general is concerned, it’s actually a category mistake to think it is in the brain (see eg. previous post). The implication is that functional mental illness cannot be reduced to brain disease in principle (see eg. another previous post). More generally, life cannot be explained in terms of mechanical principles of nature (see eg. yet another previous post).
The first cortical localisation that became widely accepted was linking speech to the frontal cortex. Paul Broca associated damage to the frontal cortex with aphasia in 1861. Nine years later Hitzig and Fritsch discovered the dog's cortical motor area from their observations in dogs after a variety of cortical lesions. Friedrich Goltz (1834-1902) was, however, certain that intellect could not be confined to discrete parts of the cerebrum. He rejected physiological reductionism and believed dementia was a function of the whole cerebrum. Sensory and motor functions are localised to some extent but not all human functions are localised in the brain.
The brain lesion literature continued to support the idea that mental illness could be due to pathological changes in higher structures. This led to Moniz being awarded the 1949 Nobel Prize for Medicine or Physiology for his leucotomy procedures. The modern era of psychopharmacology began believing that chlorpromazine caused an equivalent chemical lobotomy. Psychiatry has still not developed from the era of psychopathological structures (see previous post), despite the disaster, for example, of Walter Freeman’s icepick leucotomy (see eg. my OpenMind article and book review).
It suits psychiatry to forget all this history. It continues to fudge the difference between functional and organic mental illness (see eg. previous post). This is despite it learning at least by the beginning of the twentieth century that human function is not as well localised in the brain as we might hope or expect. Nor should it be conceptually (see eg. another previous post). It’s about time psychiatry moved on from its outdated physical model of mental illness.