Sounds like you’ve never worked on a game. All software is developed with some level of trial and error, and if you were really a software engineer you’d know that.
Games involve art design, sound design, systems design, and game design. Most software involves a single design philosophy trying to reach a specific objective. Software you can usually take a look at early and with caveats get an idea how it will work together. Games you can’t do that with so easily - that’s why there’s a specific term for it called a vertical slice.
You wouldn’t need a vertical slice if you could just look at any point of the game being developed and get an idea where it’s going.
This doesn’t even touch on the fact that you often can’t be sure if a design will work or will fall flat, software is generally less volatile than that.
I like how unapologetically purple they are