Same reason Google forked webkit to make blink. Over time the involved parties' priorities diverge and when that gap is wide enough, it makes sense to fork.
Also note it's not just Edge we're talking about. Microsoft is one of the main Electron stewards, and products like VSCode are built on that platform.
As I said, I'd like that to happen, but I don't see MS being bothered by diverging priorities enough to fork after _throwing away_ a whole engine to avoid duplicating work. At least not in the medium term.
I don't have any insider knowledge. It makes sense to me that a trillion-dollar platform company will at some point want to control their browser destiny, and not rely on their competitor's roadmap.
You may very well be right. I have no special knowledge of internal workings at MS. Having said that, I can guarantee that there are factions within MS that were against getting rid of Edge and factions that are all about MS building their own browser. It's very possible these factions may win out at some point in the future. They certainly have the talent and resources to work on Chromium/Edge independently.
Also note it's not just Edge we're talking about. Microsoft is one of the main Electron stewards, and products like VSCode are built on that platform.