Because it is not a weapon. It is a tool added to a weapon to help control the audio output of the sound the weapon makes. Silencer is actually an incorrect statement because it only helps control the output level of the sound, making it less lethal to everyones hearing. The sound output is only cut in half usually, but that is enough to help normal hearing helpers to work better. Don't believe the silencers of Hollywood, as those are not real guns and are not actual silencers.