Disclaimer: I have no qualifications or really any business talking about this...

I think games aren't the best kind of projects for open source. Some games are made open source after development ends which is cool because it opens up forks and modding (pixel dungeon did this). Most games require a single, unified, creative vision which is hard to get from an "anyone can help" contribution style. Most open source software are tools for doing specific things. It's almost objective what needs to be done to improve the software while games are much more opinionated and fuzzy. So many times I've seen a game's community rally behind a suggestion to address a problem and the developer ignores them and implements a better idea to more elegantly solve it. Most people aren't game designers but they feel like they could be.

An exception to this are certain, rules-based puzzly games. Bit-Burner is an open source hacking game with relatively simple mechanics and it works well.