A few things come to mind. First and foremost is a lack of breadth in intellectual thinking. I live and have lived on both sides of the intellectual street. In the rarified halls of academia thinking tends to be much more open and accepting. Most academics would agree with the statement that, “A Ph.D. doesn’t mean a damn thing, unless you haven’t got one.”
Academics are acutely aware of their limitations (at least around their colleagues). Your link to the meaning of a Ph.D. is not only funny it is quite accurate and the vast majority of people in academia would appreciate it and would pass it around for the humor and the truth that it contains. However, academics can be equally as rigid.
Kuhn’s theory of a Paradigm Shift
http://en.wikipedia.org/wiki/Paradigm_shift is one way to explain much of the rigidity that we see in the world. While there are several corollaries I like the idea that people grow up learning a particular way. As they mature they use this way and find it to be successful. This increases their commitment to a particular view. Later, the way is replaced with a new way that they have not learned, do not understand, and do not want to take the time to learn. To justify their commitment the new way must be devalued and the old way is better in their thinking. To have a revolution in thinking we have to wait for the old guys to die off.
Within the individual the same thing occurs. If you learn to play 8-Ball and you become reasonably good at it, then you develop a commitment to 8-Ball and it becomes the only game in town. Anything that is not 8-Ball related has to be devalued and this proves your commitment to the great game of 8-Ball. This devaluation includes, people, places, equipment, etc. The more you defend your idea the more you have to defend you own statements and defensiveness gets even stronger (see the resolution of dissonance
http://en.wikipedia.org/wiki/Cognitive_dissonance).
With maturity and a modicum of intellectual ability we begin to realize what we have done to ourselves and we begin to branch out in our thinking. In my opinion this is one of the primary functions of a college education, to expand one’s ability to think and truly see the gray in the world.
Of course some people regardless of age, intellectual ability or education simply have emotional problems that tie them to a particular world view. They are of course limited by their emotional problem and that is a different topic indeed.
PS, It takes four years to get a liberalizing education. Of course there are other ways and some people who do attend never learn to expand their mind.