Here's why:

* Amplitude Definition: Amplitude is the maximum displacement of a wave from its equilibrium position.

* Zero Displacement: If the displacement were zero, there wouldn't be a wave at all. It would just be a flat line.

However, there are a few things to consider:

* At a specific point in time: The displacement of a wave at a specific point in time can be zero. For example, a sine wave crosses the equilibrium line (amplitude = 0) at regular intervals.

* Zero Amplitude for Certain Waves: Some waves, like shock waves, have a very short duration and their amplitude might be considered effectively zero after a very short time.

* Amplitude Related to Energy: The amplitude of a wave is directly related to its energy. A zero amplitude wave would mean zero energy, and therefore, no wave.

In summary: While a wave's displacement can be zero at specific points in time, its amplitude itself is never zero because it represents the maximum displacement, which is a fundamental property of the wave.