"One place the open source ethos has lagged, however, is in the games themselves. Many open source games exist, but it’s still far more common for developers to use open source tools to build proprietary games. “I often see people releasing little parts of games, like helper functions, and the examples from game development tutorials are often available on GitHub,” says Phaser creator Davey. “But it’s still much less common to see people sharing complete game code.”
“I feel a little guilty about not open sourcing Monstrüous,” Millot says. “It took me sweat and tears to make. I’m proud of it and I wouldn’t want someone to use my art in some way that I wouldn’t like or to sell it or something like that.”"
Article does not mention the greater challenge of getting people to release game multimedia and level campaigns as libre.