Due to decades of work reverse engineering the behavior of MS Office (not only after the invention of DOCX, many of the specified behaviors in DOCX reference behaviors of older versions of Office).
It would be a monumental effort to create another implementation and completely impossible without referring to MS Office as a reference implementation.
It's completely unrelated. LibreOffice can open most completely binary DOC or XLS as well, they were just reverse engineered. It has nothing to do with openness.
Correlation does not imply causation. The fact that Libreoffice can open binary proprietary formats does not automatically imply that all formats it uses are binary and proprietary. DOCX is an open standard.
DOCX is not an open standard, there are many articles[1][2] why it isn't - even in Wikipedia article [3]. ISO standards committee were just a rubber stamp Microsoft puppets to ensure vendor lock for decades.
I stand corrected, sorry. Emulating some other proprietary software really shouldn't be a paragraph in something we call an "open standard". Thank you for the links.
It would be a monumental effort to create another implementation and completely impossible without referring to MS Office as a reference implementation.