I don't really have the numbers to back it up, but I think if you compute the expected value of starting a venture, the numbers will show you that you that taking the risk is not worth it (vs. getting a normal salaried job), since most ventures fail.

With that stated, I think that entrepreneur overconfidence is a features and not a bug. The fact that entrepreneurs are overconfident may, in most instances, be to their detriment, but it also will engender great breakthroughs that have huge payoffs and make society much better. Without overconfidence, this wouldn't even be possible.