They don't necessarily need a psychologist, but the importance of the mental aspect of sport (and gaining control over it) is nothing to be sneezed at. This is neither a well kept secret nor a new concept.
I'd be interested to see you cite some case studies or some other actual evidence of these plenty of players who have received no benefit from psychologists. Maybe you could also use your level 100 access to confidential information to have a closer look to see if there are other players who have.