Mostly because bad oysters kill with some version of Paralytic/Amnesiac/Neurotoxic/Other Shellfish Poisoning (see Wikip.,) which are caused by planktonic organisms (diatoms, dinoflagellates, etc.) that require saltwater to survive. Shellfish are exposed to these toxins in the sea and can bring the toxins with them when they are harvested.
Whatever someone does to the oyster on land, such as drying it in the sun, is not going to make more of that toxin leap up out of the sea and infest the oyster.