I can answer the headline question very easily: because being smart isn't all that's required.
It's hard for people who haven't been in business to understand. They have been told that a great product at a fair price with fantastic customer service is what it takes. While that plays a role it is does not even amount to 50% of the formula.
As well as a huge amount of chance - e.g. my first company got funded because one of my co-founders ran into a guy in a bar that happened to be in the country for a week or two. Maybe we'd have found other funding if that didn't happen. Maybe not. We'll never know.
> Both IQ and personality remain as important predictors in wage equations when both are included in a regression