Unless I missed it, the article did not explain why people over estimate their intelligence. I believe it often has to do with the fact that many people live in a culture/sub-culture that offers them popular notions/dogma/beliefs that allows the adherents to be deluded into thinking one's thinking is quite correct.
I think society benefits in that many people are content in their station in life, not thinking it could be an inferior station. Therefore, no rebellion.
An important point might be that the ignorant are very content in their ignorance. In fact, many wear it as a badge of honor, in my opinion.