From the ODF:

"The alliance works globally to educate policymakers, IT administrators and the public on the benefits and opportunities of the OpenDocument Format, to help ensure that government information, records and documents are fully and natively accessible across platforms and applications, even as technologies change."

Even though it is widely used, the .DOC format certainly does not pass this simple test. Note the "even as technologies change" comment. Can you open a .DOC file created with Word 2007 in any other application other than Word 2007? Nope. Therefore it is not an "opendocument format". Can you open a .TXT file created with Word 2007 in any other application other than Word 2007? Sure can. So the .TXT format is more "open".

Interoperability is the key, "even as technologies change".

Microsoft's OOXML is only readable using Office 2007 technology, or badly translated by one of the available conversion programs. It's far from "open", has licensing restrictions, and only works reasonably well with Microsoft applications.

These and many other reasons are why OOXML is not being supported in its bid for ISO approval by any of the non-affiliated organizations.