Why? It's just another way in which a large amount of material can exist in a stable orbit around the Sun. Why is it any more incredible than having another planet there? Ceres is big enough to be considered a planet and, in fact it is a so called "Dwarf planet". It's 940 kilometers across. Do you suppose it was out there by itself and all this debris just happened to end up in roughly the same orbit or do you believe that it too is a piece of the Earth that was ejected when Noah's flood started? Do you have any concept at all of the energy it would take to launch a rock that's almost 600 miles across, even into low Earth orbit, never mind an orbit that's 3 times as far from the Sun as we are?
Perhaps you don't believe that it was sent out in one big mass, perhaps you think that Ceres is gravitationally collecting material from the asteroid belt and has grown to its current size over the last 4000 years. Such an idea borrows from theories about "cosmological evolution" where planetary systems slowly accrete material from vast clouds of gas, dust an debris. The problem with that, as I suspect you know, is that once objects get past a certain size, they don't collide and stick together anymore. They collide and smash each other back into smaller pieces.
In either case, you'd have to believe something similar, only on a very much larger scale for the Pluto, which has no similarly accompanying asteroid belt to accrete from. It just has 5 moons, one of which is nearly the same size as Pluto itself, all of which would have had to have left Earth together and principally intact in their current form.
Further, if Walt's theory is correct, there is no way that Pluto and the material in the asteroid belt accounts for anywhere near all of the material that would have gotten ejected into space. What exists today would be but a tiny fraction of that material which happen to find itself in a stable orbit. The Earth itself would have been showered for years and years with debris falling back to earth and it would have had a ring system for some lengthy period of time until all of that material was either ejected out of the solar system, or collected gravitationally by both the Earth and the moon, which there is precisely ZERO evidence for. Such evidence, by the way, would present itself as a bulge around the equator of the Moon and a large number of <4000 year old impact craters on Earth.
OR you can believe that God created Ceres, the asteroid belt and Pluto the way they exist.