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.