You are dramatically underestimating the cost of manufacturing, stocking, and distributing an entirely different SKU and probably also dramatically overestimating the number of users of alternative keyboard layouts in the world.
Put another way: I am absolutely certain that more users would get a Mac with a Dvorak layout by mistake and want to return it than would actually purchase and keep one.
Put another way: I am absolutely certain that more users would get a Mac with a Dvorak layout by mistake and want to return it than would actually purchase and keep one.