IP. To some extent liability. But really IP. It is an asset. Releasing source is giving away the asset. That's it. That is the reason. Doom's code was released

IP. To some extent liability. But really IP. It is an asset. Releasing source is giving away the asset. That's it. That is the reason. Doom's code was released because Carmack had a materially different view of IP. If you look at "open source/open core" threads for enterprise/non-game software, you can see the mindset.