Because they aren't. From the start they were upfront about the plaster casts being impressions left by their long-since decomposed bodies. The bodies carbonized, broke down, turned to dust, but the pack of ash and debris around them preserved the way they were arranged when they died. Once they excavated down to a certain level in the fill they would pour plaster in where they found the body impressions and voila. No scientist or person at Pompeii ever claimed they were the actual corpses, you're the ignorant one for thinking people were claiming otherwise.