Much like how science demystifies the world around us, teaching demystifies ourselves.
Consider the kung fu guru who claims he can kill a man in one blow. He refuses to teach you his special power because, deep down, he knows he’s bullshitting you. The kung fu guru is attached to the mysticism surrounding his sacred power. His students revere him. “Did you hear that sensei can kill a man in one blow?” Thus, he’ll never teach you how how he can just kill a man.
When we teach, we let people see that we’re just ordinary mortals who happen to be obsessed with something. It’s a humbling experience to break down our skills into concrete steps. Deconstructing our thought process, making it so that anyone can learn how to do what we do, we remove all possibility of marketing ourselves as the kung fu guru of our craft.