Well in Trump's case he was born into wealth. His father was a major real estate developer and investor, but in Queens and the outer boros of NYC. Donald Trump went into Manhattan to stake his claim and by hedging investments from other people and taking tons of loans out he was able to get credit anywhere and fund his projects. So thus increasing his net worth, even if he didn't have it in the bank/liquid, his properties and companies gave him his high net worth...
That being said, CEO's and other C-level executives typically recieve more prefered stock than most investors get. With options they typically can't sell it right away, and don't either (it makes them and the company look bad or worse, makes people think they are ditching it before it loses its value, not to just get filthy rich).
But then again, just because a company goes public, doesn't mean they are going to become billionaires overnight. Most companies that do go public statistically speaking tend to tank or don't make all that much anyway.
Finally, when these guys go on spending sprees, many times, its with the company card. You'd think they'd learn their lessons by now, but then again, most of them get away with it.