Because they're only dummies, and their only purpose is to get the layer coverage to the required level (necessary to guarantee a good yield at the concerned technological work step). Hence no accuracy for the dummies' shapes is required, and this saves CPU time for OPC dataBase calculation.
I normally hear that OPC checks are important for deep submicron technologies. But I couldnt get much info on how OPC correction is done and its significance. Can anyone explain it?