Why isn't Linus Torvalds a billionaire? (2017)

Because commercializing Linux was never the goal. There is a world where Torvalds becomes a proprietary UNIX vendor shipping Linux-as-a-service to various navies and worldwide VPS providers. But even in the 90s, all that had already been done. Much of that addressable market was already eaten or in the process of being cannibalized.

There will probably always be competing monolithic kernels out there, many owned by billionaires. Linux is differentiated because it's GNU.

Because that wasn't his goal. To become a billionaire, that has to be your primary goal. If you're doing that, then you can't have other primary goals such as doing great and impactful work.