A Word document is a bit much to be editing from the command line. If we were to access the wrapping XML document using plain text you could feasibly edit the document content, but who would ever try to do that? And how would we access the wrapper, to begin with?
Then again, who knows? If we have a Word document, then it follows we have Word on our machine. .docx opens in Word on a single click.
Alright, and how about a PDF document?, I mean, it would be easier to just open the terminal and modify it instead of opening word, wait for it to load, open the document, modify it and save it again.
Those two mediums are heavily proprietary. MS with its XML wrapper, PDF with its POSTSCRIPT wrapper. They are recognized document formats because of all they have invested in making theirs the de facto in each respect. The least we can do is use their software to manipulate the documents.