The problem is not the notch but the rounded corners.

I think they wanted to show how "cool" is the whole edge-to-edge screen with all the four rounded borders. If you black out the notch and keep that side square, then you must do the same on the other side and the screen will just look like the old rectangular display.

While I'm sure there will be some option to do this, at least for video, for me, a square shaped phone would have made sense.