such as indicators in the same color as the rear lights, are perfectly legal.

Less lights is less cost. On European streets the easiest way to detect an American-designed car is that they only have one reverse light, the bare minimum. Only suitable as an indicator to the driver behind you. Ever considered trying to reverse into a parking spot without any streetlight nearby? Reversing blind is awesome!

In any European car you get two lights, not in the center but in the corners so you can actually see stuff in your side mirrors while parking.