https://www.gnu.org/licenses/gpl-faq.html#DoesTheGPLAllowMon...
The vast majority of game studios aren't willing to do this, so effectively releasing it under the GPL means that it's much less likely that a competitor uses the code.
Asset flips from other countries with more permissive (or just hard to access for foreigners) legal systems are already a problem for game developers--releasing the source code for your game would ensure that you'd be inundated with assets flips almost immediately if your game is any good.
I don't know any examples of this happening, but we've seen the engine/assets distinction before when DOOM and Quake were opened. Their source code was released as Free and Open Source software, but the game assets remain payware to this day.