Why is the asteroid belt not a dense ring of planets?

The asteroid belt is a broad region between Mars and Jupiter containing many small rocky bodies, but it is far from crowded in everyday terms. Jupiter’s strong gravity stirred up the region and made it difficult for the material to accrete into a single large planet. Repeated collisions also broke larger bodies apart and kept the belt dynamically excited. Most of the belt’s total mass is tiny compared with a planet, much less than the Moon’s mass. Modern surveys have shown that the belt is not a solid ring but a very low-density population of objects spread across space.