Because the game doesn’t have locked framerate and the gameplay is tied to the 60FPS logic, so if you run above that you’re running the game faster. And currently even with the work around of forcing your screen to 60Hz (Something that’s not available on all screens) there’s massive screen tearing issues which makes it really painful to play.
As bad as the small amount of input delay from having V-sync enabled can be, it’s better than the alternative currently.