It is inevitable that no really powerful package will prefer to use the native file format of another different package, because their features will not be a good match. Similarly the KOffice spreadsheet doesn't want to use Gnumeric native XML files, because they won't reflect the desired features of the KDE Spreadsheet. If a standard interchange format is created, it will likely cover only the most important common features of all spreadsheets such as simple formulae and numbers.
More complicated features increase the difficulty of implementation significantly.
Both can load a lot of open file formats, such as PNG, but those don't preserve all the data expressed in the native files. Such programs on the whole don't have much use for XML, because they deal mostly in widely understood binary formats.