You might be able to make a Notepad clone in an afternoon, but not wordpad.
Creating parsers for all the formats it supports (rtf/doc/html) as well as the renderer/wysiwyg editor would take anyone several days of either coding, or stitching libs together.
https://github.com/microsoft/VCSamples/tree/master/VC2010Sam...
I'm assuming that's not built into the widget or into Windows. Is there an open-source drop-in docx<->rtf library out there?
Since there was now a RichText control that supported RTF in Windows, the Windows group could write a thin wrapper around the RichText control and replace Microsoft Write with Microsoft WordPad.
Support for Microsoft Word file formats came later once the Word group took over responsibility for the RichText control code.
No shipping code is free.