I can’t speak for them because I never worked on them, but you’d be surprised how often a piece of music or a graphic is copyrighted. One of thousands of your common textures gets a bit too close to something else and suddenly you need a license.

Fun fact: a lot of game audio is licensed too, sound effects and such.

Regardless, the issue of sublicensing goes beyond what you’re allowed to let people do, it also goes into the idea that you’re often forced to disallow people from harvesting those assets from the game, or allowing the game to turn into derivative works - and because you yourself do not actually own the asset, you’re forced to confront it.

To avoid these issues, games would have to be very small (2D? Chiptunes? idk how small), but it’s one of a million tiny issues that comes up in game development, and each one of those tiny issues risks not allowing you to release the game.

Games are really, really hard to make, there are so many issues waiting to kill it- and even if you manage to make it, there’s no guarantee it’s successful, so spending time on these things is stealing time from making it fun or viable.