If schizophrenia is a brain illness why isn't it detected by brain scans

As far as I am aware there's no biological test for schizophrenia used
in UK clinical practice. While the medication acts at a biological level
based on a dopamine hypothesis (which is probably bollocks given that
the drugs were invented to sedate people without sending them to sleep
and the drugs don't work on all people nor do they attempt to regenerate
the supposed deficits in brain matter) doctors use testing based on
symptoms of behaviour.

So when my mum showed symptoms of multiple sclerosis to confirm the
diagnosis they did an MRI and a lumbar puncture. That's how they worked
out with a high degree of certainty that she has MS even though it's
rare in Asians.

There are tests in development but currently there's a high degree of
subjectivity. This subjectivity leads to one of the big problems in
mental health in the UK and US: the overdiagnosis of black men. Many
reasons are given but none of them totally explain the high levels of
overdiagnosis nor cases of people with disorders on the schizophrenia
spectrum who leave the UK and go to the West Indies then don't have a
diagnosis of schizophrenia there.

It may also change the high rate of diagnosis in London. It's something
like twice the rate in other cities in the UK. For the life of me I
can't think why at a brain illness level. I think there may be more
stresses and there may be more need for people to be 'normal' but this
isn't relevant to a brain illness. That sort of thing would need more of
something in the air or water.