I think they ignore this for the same reason they do it in Europe. Because they need the immigrants to help fund the massive entitlement program tsunami coming as the baby boomers continue retiring and needing medical services over the next 20 years. The younger the immigrant the better because you can bleed them longer before they become a drag on the system. That and the incentive of cheap labor for big contributors.
Absoultely nailed it. This is even more true for Europe then the USA. Europe is facing a demographic disaster. Think tanks etc have looked at it and they figured virtually unlimited Immigration, rather then reforming the system, is the only viable solution. They sold that to the poltiical class in both the US and the EU. They took the easy answer rather then the correct one.