This is not the case. This question presumes that shape-shifting occupation of pre-existing organisms is the major way Reptilians continue to persist as physical beings. The opposite is truly the case. Their shape-shifting to take over a human form is a very selective targeted undertaking with full appreciation and foreknowledge it will demand their continued residence through the life of the human to maintain the subterfuge, as a sudden disappearance of a famous person, for example, would cause great consternation and alarm and would provide a clue if it were repeated again and again that something fishy is going on. So the main way Reptilians continue to propagate and carry on their lineage is through reincarnation in the same fashion as physical human, by an act of procreation that creates a new living Reptilian and allowing continuation of the species.