Because they have better resolution and better colors and picture quality.
And their not exactly superior, but preferred.
From my experience they help and their practically made for creation and editing as opposed to PC being for just about everything else.
Although I agree Macs are superior for editing, there is no such thing as a computer that has "better resolution and better colors and picture quality." All of that depends mainly on the display being used, and somewhat on the graphics processing unit, which is only a fraction of the overall computer hardware.