Because all the cycles are allocated to the verbal system rendering motor system immobile.
I almost chocked when reading this.
He moves but, is too fast for the camera speed.
No, the trick is just that he is interlacing himself, didn't you notice??
(otherwise he would flicker, obviously)
John Carmack facts!
He takes bathroom breaks during vertical refresh.
I was always impressed by this during several of his keynotes at QuakeCon the past few years. I believe it was 2012 when he was standing next to the chair talking and didn't seem to flinch for almost 2 hours straight.
Without a podium, he does look absurdly immobile on that large stage.