Here's why:

* Waves need a medium: Waves are disturbances that travel through a medium, transferring energy but not matter.

* The spring's material is the medium: The coils of the spring are what the wave travels through. The individual coils don't move far, but they oscillate back and forth, transferring energy to the next coil, creating the wave.

Think of it like this: if you were to shake one end of the spring, you wouldn't be moving the entire spring at once. Instead, the shaking would create a series of compressions and expansions that travel along the spring.