The point is when the image is not an image. Maybe it's being drawn on a canvas, or composited with multiple layers, or you want only a part of it (all of these apply to Photopea, I believe).

I don't like the way clipboard access is handled in browsers either, but the fact remains that if I'm using an in-browser image editor, select a part of the image and press CTRL+C, I want that portion of the image in my clipboard. How it gets there is an implementation detail that the average user doesn't and, if it doesn't affect security/privacy, shouldn't have to care about. And since it's write-only, there's no security/privacy concern.