Here is my cynical view: From what I know they have a history of not using their own tools. They didn't use SourceSafe, but Perforce. Then they made an effort to switch to TFS, realized that it sucks and moved on to git.
You can see the same pattern in Windows desktop apps. They didn't use MFC for themselves, didn't use Winforms, used WPF only a little.
There is a watered-down version of Office in UWP.
The built-in OneNote is actually this UWP. It uses a different stylus filtering algorithm from the desktop version, which is why I don't use it myself, but rest assured it works just fine.
To be somewhat less cynical, VSS and TFVC were not intended to scale to the size of Windows's codebase, thus they weren't used. And instead of inventing our own thing this time around, we went with (and make contributions back to) the de facto standard.
When I was in Xbox, we had a lot of things in TFVC: all the services code, most of the console and Windows apps, and many of the tools. Only the Windows-related bits were in SD.
You can see the same pattern in Windows desktop apps. They didn't use MFC for themselves, didn't use Winforms, used WPF only a little.
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