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?
Others have given good answers (The reserve army of labour) but may I give another one. They're fantasically, transcendentally stupid and they've got nothing left but cruelty.
They are running out of productive forces to centralise, have rotted innovation from within, and have gone all in on AI as their last hope. They've not so much drunk the kool aid as injected a kool aid producing gene directly into their cells. It's their last gasp as China's natural productive capacity outpaces them.
If you really believe in AI, you believe that most labour will be replaced. But, you can't have labour be replaced since it causes a crisis of consumption and there's only so many ivory backscratchers you can own. So you need a minimal UBI to maintain capital flows in the medium term (using it as guillotine insurance has only occured to a few in DMT fueled fever nightmares). Additionally you need to fuck over China by outproducing them and maintaining the living standards of the upper middle classes in the imperial core.
On the other hand, UBI costs money and you like money and your shareholders are literally legally obliging you to give them all the money in the universe. So you have the contradiction of
a) Needing to prevent a crisis of domestic consumption
b) Fucking over the third world with over-production to screw China
c) No tax, only spend.
d) If everythings a robot we don't need third world slaves anymore!
So a UBI (or more likely a bullshit jobs generator to make middle management tyrants happy) inside the core (for Pell grant recipients etc etc) and a air gapped populace outside being sold stuff they increasingly can't afford, kept in check by bombs.
Some might say, this is just Generalplan Ost with robots, to which I say "yeah, good thing AI is mostly bullshit huh?"