The reason I am not smart is because I am smart.
Nature has evolved man to really function in this complex world to his advantage. The number of neurons and the neural connections in the brain have been set for a marvelous understanding of the most complex of all natural systems. The system whose foundation built upon himself, 'the man'. A man, an individual of this system, is very complex in its working, and this complexity increases exponentially with the number of the interactions of these individuals.
Throughout history, the man knew how to behave in the complex of society ... how to love, hate, be afraid, excel, get motivated etc. The emotions driving the man are really the manifestation of the understanding of the natural complexity of man and society, albeit this understanding is embedded within.
What nature did not do is to wire him to create models of systems. A model is an understanding that is external to the mind. A model is something which can be articulated, documented, and transferred from one mind to another.
Now the advent of science is basically the newly acquired facility of making models. What the man started doing is creating models. He started with making models of natural phenomena; of simple things like the day and night, sun and the moon and the stars and gun powder. This gave new understanding, specially the one which can be articulated and transferred from one to another and allowed one man to pronounce very clearly possession of this new understanding and to point out absence of this in other men. This created what science refers to as 'the explanatory' power.
Though this new understanding was very small in magnitude as compare to what every man already understood, the articulation power and transference power gave possessors great advantage over the others. This advantage led men to a race, a race of understanding the models of more and more natural systems. Any one who can understand 'complex' models of natural phenomena started to be considered 'smart'.
The 'smarter' a person got, the greater the reliance of his understanding became on models. Which lead to a breed of very 'smart' men, those who hold great many models in their minds and those of great complexity.
Since evolution does not work on rational principles, there was some understanding which was erroneous (it can be argued that these errors would have been dropped automatically by evolution itself). When the smart man created irrefutable models and showed errors in some of the common evolutionary understanding, smart men's trust on whole of the evolutionary understanding got shaken.
Smart men started dropping their reliance on the evolutionary understanding and started relying on modeled understanding more and more. One thing the smart man did not realize was that the complexity of the models still did not reach a fraction of the complexity of the working of the man and the society. The emphasis on models made the man started loosing what he already knew, just because he did not have models of that understanding.
This is why the smart men are not 'street smart'. This is why the smart men behave awkwardly in common social situations. This is why smart men don't run countries. This is why we find Sheldon Cooper. This is why I am not smart.