I _believe_ they are doing this because they are heavily investing in Electron (Skype, Visual Studio Code, Teams, more). Having both Chromium and V8 be part of Edge (and the OS) means they can remove them as bundled dependencies from Electron on Windows and lighten the payload of those apps. I wouldn't be surprised if we see future Windows builds of Electron becoming smaller because of this.

There is not proof of this, this is just a hunch I have.

Eletrino[1] and Quark[2] tried tackling this idea of using native OS engines but they seem to just be side projects and/or experiments.

[1] https://github.com/pojala/electrino

[2] https://github.com/jscherer92/Quark