A few potential obstacles:

1. Use of proprietary third party libraries. Havok seems to cost money for example. I’m not sure how Havok would work out in an open source model, but there are probably many other third party libraries that would stand in the way as well.
2. Distribution of assets. The game is not much without its assets, and many of the assets can be third party. For example Quixel Megascans. Even old games like Super Mario 64 heavily used third party textures and sounds. Not sure how they would like their assets be distributed freely.
3. Music. If the game uses licensed music this is a no go. It’s even difficult for companies themselves to release their old games with their original music.

I'll add something which is not mentioned: Unreal Engine, one of the most popular game engines, is source available, but not open source. Many games modify it but its then impossible to release those sources. I'm guessing that it's the same with many other engines...

One obstacle may be the assets. Im not a lawyer.

But you pay for a license to use a piece of music in a commercial software. To make the game open source would imply the music is also something others can use, which wasn't yours to give away.

Music is easy to rip out.

But think about the headaches. Is this 3D model that you purchased and slightly edited licensed in that way? Is this UI for the options page? What about this sound effect? It gets even worse with code dependencies. You paid to use this library...

It's now this massive headache to itemize every single thing and determine the origin, as well as if it's something you can release as part of your open source. And if you do it incorrectly, you can get sued.

Another NDA'd thing would be console integration, theoretically you could better optimize something for the hardware unlicensed if you had reference code