People generally don't want to make games free because often 99% of what makes a game good is not the software aspect. People like games for interesting mechanics, story, art, and music. Those aren't things that generally haven't worked well being free and open

FOSS generally works because people use foss to create end products, and have an incentive to contribute because it benefits them financially (and the side effects is that it benefits others too).

Making a game FOSS rarely benefits the creators since it is the end product, even if it benefits the game or community.

There are cases where it works though, such as rhythm games, where the end product requires immense collaboration, but those often exist on the borderline of acceptability (due to copyrighted music use) and they end up with a need to be foss since licensing 10,000 songs is basically impossible.

(Shout out Quaver)