To really delve into the technical side of things: I’m fairly certain that large parts of Windows, especially when it comes to DRM, rely on third-party technologies, components that Microsoft licenses and integrates but doesn't necessarily have full visibility into or control over. That could very well include parts of the DRM stack itself.

So while Microsoft controls the platform, that doesn’t automatically mean they can trivially extract plaintext content from DRMed streams, especially not without compromising the integrity of systems they themselves may not fully own.