Note that, while VS is still proprietary, VSCode is open source.
Keep in mind that there's a considerable overhead in open sourcing large products with very old codebases that evolved over many years, sometimes decades (this describes Windows, Office and VS, among many others). For example, they may incorporate code and components from other sources, and the license that permitted their use and redistribution may not permit opening the source. Worse yet, in many cases, you won't even be sure if they do or do not before you do an expensive code audit.
Open sourcing something mostly brand new, like ASP.NET Core or Roslyn, is much easier in comparison, because you architecture for that from the get go.
Yes, of course. But they're products in a similar area, with the main difference being that one is a brand-new, from-scratch codebase, while another one is ~20 years old, and in places, might be based on code going even further back. Which reinforces my point that it's much easier to open source new stuff.
Keep in mind that there's a considerable overhead in open sourcing large products with very old codebases that evolved over many years, sometimes decades (this describes Windows, Office and VS, among many others). For example, they may incorporate code and components from other sources, and the license that permitted their use and redistribution may not permit opening the source. Worse yet, in many cases, you won't even be sure if they do or do not before you do an expensive code audit.
Open sourcing something mostly brand new, like ASP.NET Core or Roslyn, is much easier in comparison, because you architecture for that from the get go.