looking at the brain would not tell them a whole lot, everyones brain is a little difference so while there are some traits that might hint towards certain disorders it might be impossible to tell for some. The brain is a complicated place and I find it unlikely they are going to get to the point where they can 'see' all the disorders physically and diagnose based on brain scans though it might help in some cases...perhaps the same symptoms have different causes or would look different depending on the individual.

Also I imagine its very expensive to get a brain scan.