2 reasons.
1. The purpose of a DSB is to allow the sand to develop different layers of bacteria populations. The upper layer or aerobic area is oxygen rich and the baceterias are efficient at breaking down amonia and nitrite into nitrate. Further down in the sand bed, you reach an area that does not have oxygen, this is the anerobic area. The bacteria found in this area are efficient at breaking down nitrate into nitrogen. When you disturb the sand bed, you are breaking down these barriers, and forcing the bacteria to recolonize the sand bed.
2. The prescence of microfauna, the stuff that makes a live sand bed alive, is essential to a properly working DSB. On a micro level, the microfauna perfom some sifting of the sand bed, helping water diffuse down to the lower anerobic area and preventing the sand bed from clumping. (this is alot different than you or a starfish sifting the bed) Sand sifting starfish also predate on these microfauna and are very efficient at it. One in a very large tank would probably be ok, but three in almost any size tank is goint to wipe out the microfauna population very quickly. leaving you with a dead sand bed, that does not have the microfauna population necessary to function properly.
