There are other reasons for separating logic into functions besides just keeping it DRY. It's an opportunity to encapsulate concerns and then, in some other place, compose the story poetically and clearly.
...which is NOT high on the list of concerns for a game title, what you want instead is massive performance gains, so you can do more with less, and in the end produce a better experience than your competitor. -- function calls/indirection/making your code 'easy' to understand, all have a RUNTIME cost, and many small costs add up to a large cost, the reasoning is really that simple.
I struggle to see how composing into functions adds a runtime cost when using modern compilers; as others have pointed out the compiler will inline it if the function is only used once.
This rationalization doesn’t make sense to me, especially for game engines which I assume are not limited to a release or 2 but are used to power multiple titles. If the software artifact is going to live for a long time, it’s probably worth the effort to make it easier to understand and test.