Psychologists do study personality. But despite that I still think as human beings our theories of mind are based on our own understanding of our mind. Those neurons that fire in our brain emulate the behavior in our brain. So the advice that any psychologist gives is based on that. But I am not a fish, so I don't know how it is like to be a fish, a psychologist in this case.

I have just received bad advice before from a psychologist and looking back on it was because they don't know me and could not have known me in the few hours I spent with them. In order to know me they would have to go through a lot of people, and very few people are like me in personality terms. So to know me they would have to go through many many people before they had enough experience with people with my sort of psychology or temperament... to give me any sort of advice that would work with my temperament and actually add value to my life.