This behavior is reinforced by current child-rearing and educational practices as well. Think about how much emphasis is placed on blind confidence, self-esteem, extroversion, etc. (at least in the U.S.). So it makes sense that confidence would be rewarded later in life when society defines it as a key metric before then.

I recall reading a book which posited that this shift to confidence over other "interior" characteristics (e.g. virtues) was due to the post-war consumerism boom when salesmanship became a lucrative career. What makes a good salesman? Confidence.