They actually did somewhat start Edge from scratch originally. They made EdgeHTML as a rewrite of the IE 11 trident engine.

In the end they abandoned it and moved over to chromium. One of the reasons being Google intentionally breaking their sites for EdgeHTML.

I understood it as a technical limitation imposed by the changes Europe are demanding. They now have to allow different browser engines, so they can’t just use Safari under the hood for PWAs. They will need some UI and the technical underpinning to allow the browser engine to be selected.