It's a convention. And at one time it was useful as the thing that could be replicated anywhere.

Also, things that are white can be judged by the amount of light they reflect or scatter as a percentage, independent of the source of illumination. For instance, among familiar objects, a common white ceramic bathroom tile comes pretty close to pure white.

Also, a thing can't be "whiter than white" in nature unless it's fluorescent. So it represents a kind of limiting case that people can agree upon.