Software is very much an implementation of an idea or ideas that are rather tangible.

A simple recipe is not copyrightable because a simple recipe describes a very particular end product, and there's only a single recipe which can produce that end product. You can't change "1/4 cup of sugar" to "1/8 cup of sugar". Therefore, to copyright the recipe would be to, effectively, create a monopoly on that end product. Back in the old days judges still believed they had the power to prevent absurd legal outcomes, and they thought it absurd--or at least unintentional--that copyright could be wielded in such a way to restrain commerce and transfer of knowledge.

Any piece of software which, like a simple recipe, cannot be expressed any other way is, likewise, not copyrightable. Theoretically. Likewise, there undoubtedly exist many recipes which are copyrightable, presuming they're creative and expressive enough, and are not the only way to express the ingredients and method of preparation. For example, a method of preparation written in iambic pentameter, perhaps with some other flourishes for good measure. You would be free, of course, to extract and copy the simple recipe from such a work.