And the cake definition includes a lot of learned knowledge about humans, foods, rituals, pleasurable tastes, aesthetically pleasing looks; you practically are a domain expert on wedding cake already, you know what is possible, roughly how much it costs, how the difficulty scales with size, what isn’t possible, what isn’t reasonable, what options are available, how people will react, down to a very small scope of possibilities. what’s the cake equivalent for Martians who don’t eat and don’t have weddings and may not see in the human visual spectrum?

I want a thing which will be a centre of attention at an appropriate moment of their existence which their senses will experience .. an appropriate sensation to convey .. to achieve the appropriate goal of goodwill, at an appropriate amount of resource expenditure?

Does that still cover the important bit? If I say I want peace among nations, have I just done the important bit of international diplomacy, and it just needs some experts to tidy up the details?

The difference between your cake and real life programs is that business processes tend to be rife with exceptions and details — frequently exceptions that are unknown at initial design time.

The whole reason for the existence of iterative processes like Scrum and friends is that your premise doesn't hold in the world of software, because then we could just use a waterfall process and be on time and budget every time.