Here's why:
* They were solid projectiles: Stone cannon balls were designed to be solid, heavy objects that would be propelled by the force of the gunpowder explosion within the cannon.
* Powder in the cannon: The gunpowder used to fire a cannon was loaded separately behind the cannon ball in the cannon barrel.
* Impact damage: The primary purpose of a stone cannon ball was to inflict damage through its sheer weight and momentum upon impact.
While there are some historical examples of "fire pots" or "fire balls" which might have been filled with combustible materials, these were less common than solid stone cannon balls.