If you're talking about games then the answer is licensing. There are a zillion licenses around the audio stack, the video stack, perhaps the game art or the game characters etc etc. None of that licensing allows you to release the source code (and thus the source material) to the public.

For non-games it can also be about licensing (or as others have mentioned providing evidence that you used code in an unlicensed way) or about support (nobody wants to answer questions about the code).