this mostly applies to the U.S. but also most of the western world:
As Marxists we know that most policy is driven by what capital allows or within the increasingly narrow range of acceptable discourse it allows within bourgeois dictatorship
Obviously it's not a conspiracy of ten guys in a secret room but a general consensus that develops from a chaotic web-like oligarchy of money peddlers, influencers, lackeys, billionaire puppetmasters, etc
But this really, really hurts Capital. they need the influx of cheap labor or face the real threat of forced degrowth. and we know every 
including 
is trying to make it harder for people to be childless but short of forcing people to procreate at gunpoint..
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so why allow this to become a bipartisan consensus (U.S.) instead of say throwing some scraps of social democratic programs?
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or in Europe's case allowing these parties to come to power instead of reversing some neoliberal austerity?
Is this a case of anti-immigration just being easier to do vs. building resiliency into the system? i mean it's always easier to write laws crimializing stuff and throwing cops at a problem i suppose
Or something else?
Because production inside the core is not really as important to capital as maintaining hegemony, and it sees multiculturalism as a threat to hegemony. Leaving production in the periphery where currencies are undervalued relative to the dollar, and the colonialist power structures allow for superexploitation wages is a more beneficial state of affairs for international, imperialist capital than allowing for those workers to make their way to the imperial core. They still have to allow it in a limited way (who's gonna mow their lawn?) but they have to keep a tight restriction on it because it threatens the current neoliberal order.