It's perfectly feasible that some clever guy would compile the source and put the game on a warez site for people who don't want to pay. Open-source games are vunerable to hacks.
Also, say you write a "game engine" and make a free game with it and release it as GPL. Later you decided to write a commercial game with your engine. You might find that portals won't sell it because it's open-source. You also might find another developer has picked up your engine and used it to write a game extremely similar to yours and it's completely free. You're not likely to make much money with GPL code.
Also, say you write a "game engine" and make a free game with it and release it as GPL. Later you decided to write a commercial game with your engine. You might find that portals won't sell it because it's open-source. You also might find another developer has picked up your engine and used it to write a game extremely similar to yours and it's completely free. You're not likely to make much money with GPL code.