Yeah, there are two types of interaction with storage providers: import/export, and open in place.
Pages uses open in place and export. It doesn't support import. Dropbox doesn't support open in place, only import/export. Open in place is a lot harder to get right, as the storage provider has to cache the file on behalf of the app opening it, until that app releases its bookmark on the file.
It saves space and reduces duplication, but it makes things more complicated for devs.