Why is it so hard in this day and age (with functional brain imaging) for neuroscientists to finally understand a mechanism behind mental illnesses???

A lot to learn: 1) Brains are still "smarter" than the best computers at least at some tasks. If our best tool to understand the brain is a computer, we cannot quite fully understand brains. 2) We still can't count all nerve cells, receptors and connections, so we don't know the wiring. All brain chemicals (neurotransmitters) are still not worked out. 3) Mental illness comes from a lot of brain circuits & chems