Apparently there are at least these 20 reasons that smart people fail. If you want to look into why dumb people are overconfident (or the Dunning-Kruger effect),
1. Lack of motivation.
2. Lack of impulse control.
3. Lack of perserverance and perseveration.
4. Using the wrong abilities.
5. Inability to translate thought into action.
6. Lack of product orientation.
7. Inability to complete tasks.
8. Failure to initiate.
9. Fear of failure.
10. Procrastination.
11. Misattribution of blame.
12. Excessive self-pity.
13. Excessive dependency.
14. Wallowing in personal difficulties.
15. Distractibility and lack of concentration.
16. Spreading oneself too think or too thick.
17. Inability to delay gratification.
18. Inability to see the forest for the trees.
19. Lack of balance between critical, analytical thinking and creative, synthetic thinking.
20. Too little or too much self-confidence.
I wonder how many of these factors must be present before intelligence must be questioned.