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I would say because they feel more secure that way, in the end, when eastern European goes to work to Western European country, it's not like he will live safely amongst the western europeans, but african and asian immigrants there, so... it's only natural.
That's one reason.
Another is... well... Western Europeans lost their sense of community for some time now I would say, they dont care for their own countries & people of their country, so why would they create ghettoes in another country lol?
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