Earlier quoted context omitted.
Thanks for pointing that out. (Also odd formulation of the question: "Why aren't the open soure? - We made the decision to keep them closed source." The actual answer is to the next question: "we may provide premium developer services.")
After embracing open source, they appear to now be extending it...
This is an add-on to Microsoft’s open source editor... Is your concern that Microsoft has embraced VS Code, is now extending it, and has the sinister end goal of exterminating VS Code?