Mostly profit motive. Most open source software is free, mostly because it’s really hard to profit from something anyone can build for free. As soon as the source is released, someone will make a free build of it available and undercut the devs.
Devs, artists, etc all need to eat, so the game needs to be profitable enough to cover that.
It’s not like games can’t be open source, and I’ve played plenty that are, they just won’t likely be profitable.