Because mental health is medicine too. "Fake it til you make it" where you will cause minimal harm to others.

If I sell myself as the best Asp.Net developer in the world, and get hired to build a DotNetNuke eCommerce Site. Will my client kill themselves because I accidentally broke their website? Likely not.

A psychic could lead a person down a terrible path pretty easily, though.

A psychologist is trained, required, and regularly retested on their ability to spot potential troubling behaviors and report them.