Because the human brain can adapt to seeing colors in very different illuminations (e.g. from sunlight to tungsten light bulb), it means that color appearance is not absolute. Furthermore, the brain calibrates itself against other colors in the field of view, so that the color appearance is also affected by the colors surrounding the object being viewed.
All this means that the choice of how color information must be processed depends not just on the colors themselves. The ICC color profiles try to simplify this and optimize for printed output, adopting a standardized viewing condition that is appropriate for this.