The phone OSes don't allow arbitrary views to the file system. Every file you open from your Dropbox on mobile, you open through the Dropbox app which will then

The phone OSes don't allow arbitrary views to the file system. Every file you open from your Dropbox on mobile, you open through the Dropbox app which will then copy the downloaded file over to the opening application. Dropbox has full control over which files it downloads and which files it needs to keep around. On the desktop, the file manager and every single application (by virtue of their file open dialog) get access to any file at any time. You are supposed to be able to open any file in your Dropbox without going through a dedicated Dropbox app.