Because that's how just about any OS, application, library, filesystem, device driver, database, nearly everything related to computing at all, works. Reinventing the universe has never, ever lead to success. If you want even the slightest hope for non-negligible adoption of your operating system, you need to be able to interface with the rest of the world. And that's why you don't want the filesystem to be an "immuatble parameter of your program".