Carrot of Truth
War is Peace
Its safer long-term to just make your native population have more kids. Mass immigration is because they really would rather the people currently working their 9-5's, to be replaced with other people.
Well there is also the fact that while those people come to those countries they are never actually given citizenship in most cases. Which means they effectively have been creating a new serf class, This is the case in the US anyways I can't speak for Europe.