Scientists believe that objects in the Kuiper Belt formed with the rest of the Solar System. Most of the comets and other bodies in the Kuiper Belt did not migrate there from other regions.
While the Kuiper Belt might contain more than a trillion comets, it's a large region, and the density of comets is low. Because the region is so big and does not contain large planets, comets rarely collide with other bodies and remain intact. However, comets that pass close to each other can be flung into different regions, and comets are occasionally sent into the inner Solar System, where they can threaten Earth.