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

The alternative would be Microsoft ships windows with an outdated browser, or they include Chrome/Firefox, or they don't include any browser.

For me, I plan to use Edge to install Chrome, business as usual :P

Not sure that's the question when you already have edge... I think it turns into...

question is, why use chrome when edge is basically chrome but already installed on windows out of the box?

Because it is the windows default. From the Microsoft perspective they maintain the branding and can still pit features on top when they need to and direct people towards bing.

They trust Microsoft more than Google?