There is a huge number of people in the world who buy and use radio scanners. They spend millions upon millions of hours listening and checking every section of the radio spectrum. No message or information is safe from the listeners. Prince Charles can vouch for this. Project Echelon covers just about everything.
How many people check the visible light spectrum ?. A number approaching zero.
Light waves are cheap and easy to propogate, and travel at the speed of light, forever.
Aliens use street lights to communicate. Who notices or cares about a street light that appears to be malfunctioning. Compare this one to the next photo of one working properly, i.e. not taken over for interference free communications.