Why is instrospection, the quality of being able to hold a mirror up to one's own thoughts and emotions, a relatively rare human faculty, regardless of an individual's level of intelligence and culture? Why is it that some people can 'see' themselves lucidly - and others not at all?
Not that I am reflecting some well done study, but I believe that introspection tends to correlate with introversion. Extroverts may just be very focussed in what is going on around them to spend much time in introspection.
Also, to be introspective does not just mean to see oneself as others see one, if that's part of your definition. That might require a degree of humility, a la Woody Allen.
Back in the 1970's there was a whole genre of self-help/self analysis books. Those who wanted to understand themselves tended to be the readers, I believe. Many must have thought that their powers of introspection was lacking something.
But, regardless of one's ability to be introspective, we are socialized at such an early age, our values that we utilize to judge ourselves, during times of introspection, are so much part of ourselves, we really can't often see another viewpoint. If the antonym to introspection is "close-mindedness" then many people excel at that quality, in my opinion, myself included. It has been said, it has been written (Yul Brynner's famous line, as the Pharoh in the movie The Ten Commandments).