This is true. Almost all time it’s the textures and the sound files. Even more, time to time devs choose not to compress them as decompression could be a performance bottleneck. Deep Rock Galactic is good example for that, the game is 4 gb because there’s no textures in it except UI brushes. All 3D models of the game uses very clever Vertex Coloring techniques instead.

You might be right with it could be optional, but I’m guessing it will be a deployment hell since Steam’s (only platform that matters) underlying mechanisms doesn’t directly support it, and when it’s done with workarounds it becomes a convoluted process for end users - especially when you consider most users will download the full pack anyway.