Licensing is a huge issue. If the game is on consoles, that's a whole big chunk of code that has to be removed from the open source version. Console SDKs are us

Licensing is a huge issue. If the game is on consoles, that's a whole big chunk of code that has to be removed from the open source version. Console SDKs are usually under very heavy NDAs -- even indirect references to how the SDK works might get you into legal hot waters. Many middlewares, even for stuff like compression, are also usually under pretty punitive contracts. Older games had fewer of those problems, but practically speaking most well known releases in the last 25 years would have to have someone go through and sanitize the codebase, and that can be extremely time consuming. And depending on the specific game, stripping out all that stuff might leave behind just a useless skeleton of code because so many of the basic components that make up the game engine are third-party. Imagine getting access to a game's source code where the entire animation, loading, physics, rendering and many other small systems had been ripped out. You're usually not left with a whole lot of other stuff in those situations.