Comment by daemonologist

There are many open-source games. There are few large-scale high-polish open source games because it's difficult to build a business model around that. Unlike non-game software, games are ~exclusively consumer-facing, so you can't sell support or use them as a loss leader, and they're standalone applications that are difficult to monetize with online features or to use as the core of a closed-source commercial product.

That said, there are still lots of great projects out there (some of which have even had commercial success), for example:

- https://github.com/a327ex/SNKRX

- https://github.com/beyond-all-reason/Beyond-All-Reason

- https://github.com/endless-sky/endless-sky

- https://github.com/Anuken/Mindustry