The added information (depth, angles, shapes) don't contribute to the information being conveyed beyond a simple color change. Takes a lot more effort (however

The added information (depth, angles, shapes) don't contribute to the information being conveyed beyond a simple color change. Takes a lot more effort (however fast or cheap) to draw and perceive such effects than flat geometry. The audience may have spare mental cycles to throw at processing such info, but don't make them do so.