A big one is that the game part of making a game requires you to test it by running the game and actually using it. My impression of other areas of development

A big one is that the game part of making a game requires you to test it by running the game and actually using it. My impression of other areas of development is that it's much less likely that they execute the code they write as close to an end user as that. My other impression is that a lot of the unit testing advocates are coming from dynamic languages and need to runtime execute their code to find basic errors like typos. I know myself from writing in JS and Lua how much of a pain this is. The last part is that gameplay code changes a lot, its very much experimentally driven. In my experience of following strict processes like TDD it becomes laborious, hurts iteration time and is error prone because rewriting tests often introduces bugs just by itself. That said integration tests for bits and pieces of the game engine are gold.