The total mass in the asteroid belt is quite small - much smaller than any planet, even much smaller than Pluto. That means a lower gravitational attraction between the objects, and a much longer timescale for the collisions of them (longer than the age of the solar system). In addition, Jupiter perturbs the orbits.
I'm not 100% sure, but I think it's because the Asteroid belt is hardly a belt. Asteroids in the belt are spread extremely far apart, about 2 million miles between each asteroid [source](https://physics.stackexchange.com/questions/26712/what-is-the-average-distance-between-objects-in-our-asteroid-belt)
That's too far apart for the asteroids to attract one another and begin to collapse into anything considerable. Added with the fact that Jupiter's gravitational pull also affects the asteroids in the belt. It may not be allowing anything to form.
There are some planetoids in the asteroid belt, namely Ceres and Vesta, but those were probably formed early on. Now the belt is extremely sparse with each object many millions of kilometers away from the next, so gravitational clustering can't really occur.