Chromium is an incredibly complex codebase, and they change their internal API's quite rapidly, so upgrading is a lot more difficult than you would guess. For example, this is what it took for Electron to upgrade from Chromium 69 to 70: https://github.com/electron/electron/pull/15405/commits, and Electron doesn't even make that many modifications to Chromium. If Microsoft were to make a decent number of changes, they would need a whole team of people just to import Google's changes, and if they start adding major new features I doubt they'd be able to keep the two in sync at all.