This is not true even in those spaces anymore. Games these days require libraries like SDL, or (increasingly) use engines like Godot. Defense is a weird place,

This is not true even in those spaces anymore. Games these days require libraries like SDL, or (increasingly) use engines like Godot. Defense is a weird place, but open source is used quite a lot there, it's often required to do so and to record the open source consumed to produce a product. And often times, it must be commercial open source where you can get engineering support for the lifetime of the product's existence.