Because today most of our pursuits and interceptions are done at below the speed of light, having lights on the outside would only serve to identify police ships when they were basically stationary or in dogfights and things, and then they are identified by the radar. Fighter planes today have livery, but really that's not to identify each other when fighting, basically if you have to get close to a ship to see it's livery to identify if it's a friend or enemy you are already dead. It's unnecessary, it might look nice but I would consider it cosmetic rather than anything useful. Not that I am against cosmetic additions to the game, but there are many areas where cosmetic additions would be more useful.