A lady asked an interesting question the other day.  "Do aliens just hop in their space ships, come down to Earth, and walk into human bodies?
The answer is no.  It takes a lot of preparation to prepare a Soul to be a walk in, in another species, even if that species is quite similar to humans.
First plants and animals have to be taken from Earth and an artificial Earth-like environment has to be set up somewhere.  Then volunteers who are dying must be selected who have the proper personality.  Then, they have to be taken to the Earth-like biosphere where they eat Earth food, drink Earth water, watch Earth's entertainments, learn the language and customs of the people in the area they intend to enter.  Then, upon their deaths, they are transported to Earth, and put in the proximity of an operative working in some medical facility, and they wait for a suitable body to present itself for them to inhabit.
Such an operation obviously has to be planned years in advance, are no spur-of-the-moment thing.  An alien doesn't just drop down, say "Oh, here's a handy body!" and pops in.  They have to be acclimatized to our biosphere.  It takes a considerable amount of effort.