[wwright]

Let’s assume that when code is first written, the cleanliness and performance is somewhat random within a broad range.

If we want the code to be clean or performant, we will likely have to spend time iterating on and pruning the code. Let’s assume that improving performance and improving cleanliness are at best orthogonal, at worst opposing.

The project has a limited amount of time, particularly for games, which often have a relatively low roof for how much maintenance the code will need.

The project has a budget on time to spend between cleanliness and maintenance. Games need high performance and relatively little maintenance, so they are more likely to spend their budget on much more performance than cleanliness.

(Game engines meant for heavy reuse such as Frostbite and Unreal Engine would likely have a much more even split, and similar for games which are likely to receive recurring and invasive updates. I would expect Fortnite’s code to be fairly clean as games go, for example.)