Why Silicon Valley Is Full of One-Hit Wonders

The "Paypal Mafia" ( http://en.wikipedia.org/wiki/PayPal_Mafia ) seem to me a good counter-example, if we are in the anecdotal evidence range.

But more importantly, I think the whole "VC prefer serial-entrepreneur", is not because serial-entrepreneur are "alway right" but rather because their track record prove that they have a better chance than first time entrepreneur. Which is alway important when you are investing...