...because MS Office can't continue adding new features forever. They'll try, but eventually their software will plateau and stabilize into an Office suite that has all of the features anybody would ever want. And then it will take OO a few years to duplicate all of those features and then it'll be a real threat. OO will always be a few years behind, until MS Office stops changing.
I think the usefulness of introducing incompatible binary file formats as a way to force corporate upgrades is over for MS. And really, what new features that will be generally used could possibly be left to introduce? That leaves UI changes (hated by the established user base) and bug fixes (no revenue produced, so WGAS?). All MS has left is the delivery model since that can be tied to the "new" cloud-based version of MS Windows and Google seems to be all over that. But over time, the traditional version of MS Office will yield to OO because of economic and OS-dependency issues. Microsoft needs a new business model or it will die.