Private or President: Candidates Should be Subjected to Psychological Testing

A private in the Army has to pass a psychological battery of tests before she or he can serve.  Shouldn’t a Presidential candidate also meet certain minimum psychological tests?

So why don’t we insist such tests be administered to candidates before they are nominated by their respective parties?

Why do we demand psychological stability in the lowest ranks of our military but not in the person who may decide to launch nuclear weapons?  Is America completely absent common sense?