Shall we reword this question?
“Why do some people have so much confidence in their intelligence?”
Well, don’t you think people of high intelligence would have a lot of confidence in their intelligence? Don’t you think someone with athletic, artistic, musical, literary, mathematical, or any other talent would have confidence in that particular talent of theirs? Isn’t it logical that they would?
I suppose I should ask you why you are even asking this question in the first place. The answer is quite obvious. People with talents and gifts for superior performance for whatever their talent or gift is for. Success breeds confidence. A high IQ person will excel in many areas that require high intelligence. Hence, he will become quite confident of his intelligence. Doesn’t that just seem logical?
If you want to argue about whether it is appropriate or not to discuss your IQ score, that is another matter altogether. American society is extremely anti-intellectual and has long hated smart people. This is why we hate it when people bring up IQ and why discussing IQ is a social error much of the time. You need to be very careful how you discuss your IQ score if it is high and you know the number. It can be done in such a way that it is socially appropriate, but that’s pretty hard to figure out, and you need excellent social skills to determine that.