Everyone else is using it to write documents, email around drafts, collaboratively edit via OneDrive; sure you can use an alternative, but nobody will want to see a format but .docx, so your alternative is almost guaranteed to be worse than or equivalent to Word. If your alternative stuffs up the document's formatting you become seen as a pain in the neck. So it's just easier to use Word.
Every contract I've received or sent for red-lining uses Word and change tracking. I'm sure exceptions exist, but every lawyer I've dealt with uses Word as the defacto standard for contract negotiations.