I don’t work for MS but I think the reason why they did not, it was hard to do, not because they didn’t want to share.
That’s quite complex UI and UX in that control. Very likely, the office implementation is coupled too tight with the rest of the office code, i.e. very hard to refactor into a reusable control. The office is cross-platform, their implementation of ribbon has to be designed to work on OSX too, contributing to the complexity.
Given just what was visible to Office Extension Developers ("RIBBON XML", the giant pile of C++ MACROS, etc), the original Office Ribbon control was probably quite a complex beast.
On the other side: the new "Simplified Ribbon Control" that is in now in OneNote (the real one, not 2016) and Outlook (and eventually the rest of Office) is supposed to be the exact same code as the UWP/Fluent Ribbon that is eventually going to be open sourced into the WinUI library.
That’s quite complex UI and UX in that control. Very likely, the office implementation is coupled too tight with the rest of the office code, i.e. very hard to refactor into a reusable control. The office is cross-platform, their implementation of ribbon has to be designed to work on OSX too, contributing to the complexity.