I don't think that most of the successful startup founders I know are narcissists. They can turn on the confidence when necessary (eg. to close a negotiation), but for the most part they're quite humble.

I think that startup founders you hear about often are narcissists. Founding a company isn't really a narcissistic thing to do, but seeking press (particularly for yourself instead of your company) is. Thought experiment: how many times do you see Dropbox in the press as a product, vs. how many times do you see Drew Houston as a founder? And compare that to how often you see Mahalo as a company vs. Jason Calacanis as a founder.