Question

Why are the stringed instruments like guitar provide with a hollow sound box ?

Answer 1

In the case of the guitar string mounted to the sound box, the surface area of the sound box is greater than the surface area of the string. This means that more surrounding air particles will be forced into vibration. This causes an increase in the amplitude and thus loudness of the sound.
Hence, the stringed instruments like guitar are provided with a hollow sound box to produce loud music.