ELI5: Why aren't mental illnesses diagnosed by measuring neurotransmitter levels in the brain?

Why isn't there a way to measure levels of neurotransmittere in the brain?

Let me explain what I mean.

For many mental illnesses such as depression and anxiety, the cause is assumed to be abnormal levels of neurotransmitteres (e.g. Dopamine and Serotonin) in the brain. It would logically follow then, that the way to diagnose such illnesses is to measure the level of these neurotransmitters in the brain and compare them to normal levels, basically like any other disease is diagnosed.

However, this is not the case for mental illnesses. They are diagnosed via the often unreliable method of assessing symptoms and eliminating other causes. Why is that the case? Are there no ways to measure neurotransmitter levels in the brain or do we not have enough information on the "normal" amounts of these hormones?

Thanks in advance!

EDIT: Thank you so much for all the responses! This has been very educational. I'm going to research mental illnesses more since their causes and pathophysiology seem to be a very interesting topic that's yet to be fully uncovered.