Why make them reproduce as if Deathclaws ,Aliens ,Dunwich horrors were not enough .Perhaps it was a different strain of the FEV that they were using on the deathclaws ,or a mutation .I dont really trust the Fallout lore .Also human brain is much more complicated than those of a cameleon. So maybe it has to do something with that .After all FEV was a miliatary project to make supersoldiers ,so any kind reproduction or someting like that was out of the question .