I grew up in Muncie, Indiana, a town with a long (for this country at least) history of industry and manufacturing. My grandfathers and their fathers worked in various factories, most using large presses for making metal parts from sheet metal. These machines use immense force and finely machined dies to form and cut the product.
Whenever I heard the phrase “the die is cast,” I always assumed that’s what it referred to. The meaning was that something occurred that would shape all subsequent events as a die shapes its product.
The other day while reading, I came across the phrase and realized that’s not the derivation at all. It comes from gaming, not manufacture. A die, in this case, is the singular of dice, and cast is another word for throw.