Here's why:

* Amplitude: The amplitude of a wave is the maximum displacement of a point on the wave from its equilibrium position. It's essentially the distance from the "middle" of the wave to its highest or lowest point.

* Height (or Wave Height): The height of a wave is the vertical distance between the crest (highest point) and the trough (lowest point) of the wave.

Since the amplitude measures the distance from the equilibrium to the crest (or trough), the total height of the wave is the distance from the crest to the trough, which is twice the amplitude.