Facebook may be a big conglomerate these days, and they may not be the cool kid on the block anymore, but that doesn't mean that they're idiots. If anything, they treasure the presence of someone like Carmack all the more, because it's no longer reputionally-free for people like him to come on board. I'm sure that Facebook is appreciative when he blogs on their platform, but it's patently ridiculous to pretend that anyone would -- or could -- force it.
In fact, Facebook has already been forced to pay at least dozens of millions of dollars essentially as a direct consequence of employing Carmack [0], and I'd be willing to bet they'd happily pay (at least) dozens of millions more.
If you want your mind blown even harder, look up some of the paperwork around the Waymo / Uber debacle [1]. SV companies pay big bucks for this type of talent, even the ones you haven't heard of, like Anthony Levandowski.
If the roll of the dice is in your favor, SV stardom is at least as valuable as Hollywood stardom (and, for the most part anyway, you still get to walk around in public unharrassed).
Disclaimer: I have no special connection with any of the companies or people mentioned, I don't know the inside baseball, and this is all conjecture. But at least it's better supported than yours. ;)
[0] https://en.wikipedia.org/wiki/ZeniMax_v._Oculus
[1] https://www.cnbc.com/2017/04/03/waymos-uber-lawsuit-reveals-...