When you move, en mass, there's no impetus or drive to integrate.
If you come to a country alone, or just with your family, surrounded by natives. You're going to assimilate extremely quickly, picking up local culture and language, because you're not going to survive otherwise. Or you'll find it extremely difficult. Your children will adapt and learn even quicker.
If you come with 1,000 others, not even the 1,000,000 that have come to Germany, just 1,000. You already have a community. The people that can speak, can start businesses for you. You can work for them. You don't have to learn a new language, or adopt a new culture. You can stay the same as you were, but in another country.
The universe has a thing of tending towards the path of least resistance or lowest energy. You will find it in electronics, chemistry, physics, whatever. Humans are no exception.