I am not sure that game industry has a religion about proprietary software, they use a lot of open source libraries. But you need a lot of tools to make a game - GUI editor, animation graph editor, audio editor, cutscene editor, AI editor ... and it makes harder to create games if all of these editors are from different 3rd party providers with different controls, settings, installers, .... What is more some of these tools need to be taylored to the specific game or integrated too tightly with other editors. E.g. cutscene editor must be tightly integrated with scene editor, or AI editor must be taylored to specific game, since they use extremely different AIs.
What is more thare are not really FOSS tools for game industry. There is GIMP or Blender, but most artists prefer commercial alternatives because they are arguably better. Tiled editor is pretty popular for 2D. But other than there is almost nothing. I would love to avoid implementing an editor/debugger for my behaviour trees, or visual editor for shaders, but there is simply no good FOSS for that.