That's just how evolution works- something that already exists and is driven to stay alive is more likely to pass on its genetics than something that is not driven to stay alive. This fact has nothing to do with the philosophy of consenting to exist in the first place.
Edit: missed your first question. Something that does not exist cannot desire.
Because the typical standard of consent is that in order to do something to someone, you should have informed consent. If you cannot obtain that, then you do not do the thing. Something that does not exist cannot give informed consent, therefore you should not do the thing.