—“Why do so many high IQ people amount to nothing?”–
Because conscientiousness and ambition are equally important legs on the step stool to accomplishment.
It is not actually enjoyable being exceptionally smart after grade school. Exercising that intelligence in the marketplace (world) produces competition that results in interpersonal frictions.
The joy in life is largely the result of cooperation with others in the absence of stress.
Smart people have many options for working with others in the absence of stress and enjoying life’s journey.