We now know based on his numerous biographies and documentaries made about him that he overpromised because else his companies would not get the badly needed funds/support/customers. Is this fraud? Probably but he got away with it because he delivered enough tangible results to get people to back off. Otherwise he'd probably be in jail now.

Why does he still overpromise when he does not need to? Well the latest biography sorta explained it: He can't help himself, he needs the thrill of an ever more difficult problem to solve. (ie. he's psycho)