Basically, it's a function of evolution, particularly for men. Back in the hunter-gather days (most of human history), unjustified confidence and ruthless greed didn't work most of the time... But when it did -- it could work out big (very asymmetric payoff). For that reason, it was the winning strategy from a gene pool perspective. So basically people (or at least men) have been bred for this bad behavior... so that's why they do it... or they do it because it works, even in modern society -- think about politicans, CEOs, Wall Streeters -- most of them (not all) suffer from delusions of their own grandeur...
