Well there is, and that's called 'They won't let you onto the App store if you link data between apps directly on the device'. Plus there's legitimate concerns about malicious behavior if you don't stay confined to your pre-determined sandbox. But this is one of my biggest annoyances as a developer, and I can't wait for a mobile device that will let me write scripts freely that allow me to directly interact with the file system and manipulate those files however I wish.