Here's why:

* Mechanical waves require a medium: Mechanical waves need a substance (like air, water, or a solid) to travel through. This medium is what vibrates or oscillates as the wave passes.

* Energy transfer, not matter transfer: The wave itself is a disturbance that propagates through the medium. The particles of the medium move back and forth, transferring energy, but they don't move along with the wave.

Think of it like this:

Imagine a line of people holding hands. If you push the first person, they'll push the next, and so on, creating a "wave" that travels down the line. The wave itself moves, but the people (the medium) stay in roughly the same spot.

Examples:

* Sound waves: Sound waves travel through air, water, or solids. The air molecules vibrate, but they don't travel along with the sound.

* Water waves: Water waves are created by the oscillation of water molecules, but the water molecules themselves don't move far from their original position.

Electromagnetic waves, on the other hand, do not require a medium and can travel through a vacuum. They can transfer energy, but they can also transfer momentum, which can interact with matter.