Comment by eigenspace

My guess is that both Apple and Microsoft people see this as a tradeoff.

If the anchor point for window resizing was more inside the window, then you encounter an annoying problem where youre trying to click or drag content, but you end up just resizing your window instead.

The obvious solution is to just keep the old bezel that separate the content from the scroll wheels / resizing handles and make it visually obvious what you're doing, but apparently they think that's too ugly.