Privacy. Usability. Freedom. Performance. Compatibility. Memory footprint. I mean I could go on but..

Games are propietary. There are dozens of open source games, yes, but they all, collectively, by using reason, are propietary due to the exceptions being tiny and usually badly orhestrated attempts at games.

Realise that I hate capitalism just as much as anyone, but developing games with open code is (from a professional game dev viewpoint) a truly honorable exercise in futulity. You need to understand the ratio between making the hardest form of art possible to how the gamers view the medium and disregard any Impulse of changes to the design propagating and potentially ruin the fun in one of the abundant pitfalls is enormous. Rabid fans already disassemble every choice you make and your compiled code to "win" is not the best theater for making the code public. Add to that the intense pressure of the development cycle and you have easily made a torture device for the most passionate