“Dynamic range” doesn’t mean the range itself is changing moment to moment. In this context, “dynamic” refers to the range of signal levels or light intensities involved. In photography, that means the span of luminance or reflectance from shadows to highlights.

A more precise way to think of it is as a ratio between the darkest and brightest reproducible values, not simply a subtraction difference. Because it describes how large that tonal span is under varying real-world conditions, “dynamic” is the established term across fields like photography and audio.

So the short answer is: it’s called dynamic range because it refers to the range of light intensities (the scene’s or sensor’s tonal power/signal levels), not because the word “range” is dynamic in contrast to “static.”