Why the belt is rubble
Why didn’t the asteroid belt turn into a planet?
There was never enough rock, and Jupiter never let it gather. Everything in the belt together is about 3% of the Moon’s mass. Collisions smashed rather than stuck, because Jupiter’s gravity stirred the region. It is leftover building material, not a planet that broke. Switch it on in the asteroid belt view.
Not enough, and too stirred
Add up every asteroid and you still do not have a planet. Ceres, the largest, is itself a dwarf planet and only about a third of the belt’s mass. A planet needs material and quiet. The belt had little of the first and none of the second.
Jupiter is next door. Its tugs sped things up and changed their paths, so when two bodies met they tended to shatter instead of stick. The same gravity still carves empty lanes in the belt today.
Get more on this
The belt used to be heavier
Models in which the giant planets migrated throw most of the original belt away and mix in bodies from elsewhere. That would explain why the belt holds both dry inner-system rock and water-rich outer-system material. How much was thrown, and when, is not settled. What is left is the quiet remnant.