Because uncertainty is so repulsive!
We already saw from here that confidence is very seductive. But the looming question is, why?
Well, I reckon its because of this, as I posted on before, Ambiguity Aversion: we don't mind risk but we hate uncertainty.
Confidence is nothing more than the communication of certainty. And we love certainty, as demonstrated by the age old test, what do you want, a garaunteed £500, or, I flip a coin, if you win you the flip you get £1000, if you lose the flip, you get NOTHING. ( most people chose the garaunteed £500 )
A bird in hand is worth two in the bush.
blah blah blah.