Here's why:

* Volume is the amount of three-dimensional space occupied by a substance or object.

* Length is the distance between two points in a single dimension.

Volume is derived from length by cubing it. Think of a cube:

* The volume of a cube is calculated by multiplying its length, width, and height.

* Since all sides of a cube are equal, the volume is length * length * length (or length cubed).

So, while volume is a fundamental quantity in its own right, it is ultimately based on the fundamental quantity of length.