Those people you mention may be examples of the exception, not the rule. Certainly many new businesses work out when they are founded by someone from a wealthy family; it would be strange if that was never the case.

But maybe it's more common that successful businesses are started by founders coming from more modest means. Page & Brin, and Jobs & Wozniak come to mind (none came from poor families, but they weren't rich either). I'm sure there are many other examples, and those are just the famous ones; there are many successful companies started by people we've never heard of.