Here's why:

* Mechanical waves require a medium: Mechanical waves are disturbances that travel through a medium, like air, water, or solids. The particles in the medium oscillate or vibrate, transferring energy from one point to another.

* Vibrating source alone isn't enough: If the source vibrates in a vacuum, where there's no medium, the vibrations won't propagate as a wave. The source will just oscillate in place, without transferring energy through a medium.

Example:

* A tuning fork vibrating in the air will create sound waves (a type of mechanical wave). The vibrations of the tuning fork cause the air molecules to vibrate, creating a pressure wave that propagates through the air.

* However, a tuning fork vibrating in a vacuum will not produce sound waves. The vibrations of the tuning fork will be confined to the fork itself, and no sound will be heard.

In summary:

A vibrating source is necessary to initiate a wave, but the presence of a medium is crucial for the vibrations to propagate as a mechanical wave.