Office is a completely separate team divorced from Windows proper. Unless Office deemed it wise to rewrite their UI, they're not going to do so (and it's a frankenstein of a Win32 UI).

Office on Windows relies heavily on COM and other Win32-only libraries to function.

I can't think of a valid reason to rewrite Office to that extent. They already have Office for the web and Mac Office; while not identical in features, they're often good enough outside of BI scenarios or highly complex Excel work.

Outlook is the lone exception where that team decided to have Outlook for the web, Windows Outlook, and Mac Outlook be identical, so those are getting their rewrites with removal of Win32-specific features where applicable.