Well, you know, it's all about investments in...things. Running a company that does stuff. Manufacturing or something. And yeah, they're not thirty yet, but he's (it's almost always a guy) smart and charismatic. Isn't that all it takes to be rich?
Let's not forget that these companies run themselves, and readers rarely get to see what it means to be a CEO, aside from chasing after a plain-faced brunet.