Not only that, the recourse of inspecting and modifying the code isn’t realistic to most people. Many open source projects die just because literally nobody with the skills required to maintain them are interested.
Then you get to certain types of programs where open source provides no benefit: specifically, games.
Games are largely just artistic content. Making games open source essentially means that all the content is going to be taken without compensating the artists. Open source works for other programs because they depend on interoperability. But games are just content, like a movie or book. The source is the product. For other programs, support and continual development are the product.
And doing so has lead to a number of ports that wouldn't exist otherwise.
The artists are still compensated fine. Because they separate the engine from the assets. If you want to play the original game, fine, but own it.
But now the community gets to port the game easier to a new platform. Which is one of the reasons that it's easy to port DOOM to every platform out there.
Translations between the APIs (Carbon to Cocoa, OpenGL to DirectX) might be very hard, but at least upgrades would be possible.