Because it is a video game, which are usually not OS, and is available on all platforms. It it was OS, you would know, and the maker wouldn't be charging for it on Steam.
If we're lucky, we'll see some open source on this, but probably only in as far as it directly makes the game more fun. Or harder. You can likely mod the game quite a bit though, as the Content folder is full of plain text code. (I'm assuming .cso files are compiled glsl code?)
And thanks for the reminder for Quadrilateral Cowboy; I have no idea why I don't have this in my library yet. I don't think I've heard anything negative about it from anyone, and it looks like an absolute blast!
Somebody might build an OS emulator of the "hardware," as did happen with TIS-100