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?
Several things come to mind, here's a few.
In global capitalism the owning class has access to this cheap labor already, it's just out of sight in the West. The system prefers this because this justifies the low wages and can be used to lower wages in the core as well. This hides the oppression from the Western consumers and keeps their consumer power high enough to buy all the treats that creates the profit for the capitalist.
So I think that the walled garden of consumers and higher prices is needed to uphold the divide of consumer status and collection of profit.
There are also several entire industries in the service sectors in the core that manage and control the labor surplus. Which creates bs job type finance movement and policing etc. Not to mention how borders and border control create this "profit from nothing". The service sectors of post industrial countries are huge and they all rely on these divides to justify their existence.
If migration was free it would also reveal to the masses the pointlesness of national borders. It would bright forward a call for fair wages on a global scale. If they could no longer outsource labor, but the labor would come to them, there would have to be higher wages or there would likely be revolution.
There are also many unemployed people in imperial core countries that at the same time do brain drain colonialism from the global south. There isn't a lack of laborers, but a lack of "skilled labor" in the service sectors because there is no industry. The industry is in China and elsewhere now where there are plenty of people to do the work. We just have the service sector stuff now and some low wage jobs that nobody can be forced to do, not unless oppressed under the migration apparatus or the status of modern slavery of being paperless in one of these countries.
I also think that since access to the core is presented like some coveted achievement that people are forced to fight for, it creates a system that just re-enforces itself.
And people with no papers or status can be exploited uncheched inside the core as well and this too requires the anti-migrant sentiment and controls to exist. If everybody had a right to be there, those everyone would be a lot harder to exploit.