Making billions and being good at programming have nothing to do with each other. Being rich is mostly about getting the idea right and paying others to solve the real problems. Facebook is a good example; Zuckerberg made a nice prototype, but to scale, he had to bring in the kind of people that write their own compilers. He gets all the money because he came up with the idea. But the people that made it work were the programmers.

Also, Carmack is famous for the algorithms he's designed, not for using DirectX.