If you're a classically-trained programmer, or a hacker who learned from classical programming texts or the effluvia thereof, you probably think of programs in terms of algorithms, and you think of a program as being concerned with inputs and outputs.
I think this is why gameplay programming can be so mysterious to a lot of people, because gameplay programming is traditionally not at all conceptualized in this way. Gameplay programming is all about update loops; there's no formalized inputs and outputs, there's simply "what changed since the last frame, and how should I change the state now which will carry over into the next frame", which is a very different way to think about programming. the game is a very gnarly data structure living in memory and the code's job is to keep it changing frame by frame in a way that adds up to game mechanics, even though in a traditional codebase those mechanics are only expressed through frame-wise incremental changes
