Suspend a string between two points, pull it tight and then snap it with your finger. You'll hear an audible sound. Take a similar string, mount it to a guitar and then pluck it. Again, you'll hear a sound, but this time it will be much louder.
You're experiencing an aspect of forced vibration. The greater the surface area of an object you strike or strum, the more it makes contact with the surrounding medium, such as air. It's called forced vibration because the air is being forced by the instrument to vibrate at a frequency that's not its own.
Musical instruments leverage forced vibration to make sounds louder than they'd otherwise be. A piano uses a sound board, and a violin has a hollow body attached to the fret board. Both help to increase loudness for listeners.