Possibly because a bar has many factors that influence its volume: flatness of each side, parallelism of edges and sides, sharpness of each edge and corner, length of each side, etc. A "perfect" sphere has only one factor, its radius.
“A “perfect” sphere has only one factor, its radius.”
Actually, a sphere has not one, but rather it has innumerable sides, the exact number of which depends on the resolution of your measurement system. It is far more difficult to make a perfect sphere than it is to make a perfect cube or bar because of the difficulty in measuring and referencing the location of all points on a sphere. One can measure a flat-sided surface fairly easily because it is far easier to reference a flat surface than to reference a sphere. It is also easier to shape and polish a flat surface than to shape and polish a sphere.