Both Firefox and Chromium have battle-tested cross-platform sandbox implementations, but I imagine it'd be laborious to integrate them as a third-party dependency to other projects. Why not spend resources on repackaging them in an easy-to-use SDK instead of reinventing the wheel?

Microsoft is already a Chromium contributor so it's not like they lack in-house expertise or something.

https://wiki.mozilla.org/Security/Sandbox

https://chromium.googlesource.com/chromium/src/+/HEAD/docs/d...