At bottom, it's politics. Most people in Asia, Africa, and the Middle East have a top-down, paternalistic view of government compared to western europeans and in particular Americans. So political parties who support a paternalistic government are heavily incentivized to increase immigration to gain voting power over their political rivals. It's basic math.
That's the type of comment people make when they don't want to understand what the others side's actual views are, so instead they tell themselves a simple story they can believe rather than engage with the actual reason.
Both sides do it, but that's a prime example of one of the popular ones from one side.
It's Occam's Razor. In particular, nothing else explains why immigration became such a major issue for left-leaning parties around the western world only in the last 15 years: https://www.npr.org/2019/02/19/694804917/democrats-used-to-t.... As it became apparent that immigrants were a deep well of votes for people who wanted to expand the welfare state, immigration--which the left traditionally had viewed through the lens of economics and labor markets--became reframed as a civil rights and moral issue.
But the people in for example the Biden administration didn't consider themselves authoritarian or particularly paternalistic. They saw themselves as in a fight against structural oppression.
Although I agree with you that non-white immigration has had the effect of making the electorate more paternalistic, my guess is that most on the Left would be disappointed to learn that.
The pro-immigrant Left must believe that the immigration helps in the fight against oppression somehow or that it helps the immigrants while not harming the US. I think particularly that they reject (as racist) any suggestion that non-white immigration might harm or diminish the US in any substantial or lasting way.
Some of the comment on HN over the years claim that immigration is essential to keep the US from collapsing (although I don't know whether that view has any influence among Washington decisionmakers). If someone believes that US whites got most of their wealth and privileges by extracting labor from non-whites, then it starts to make sense to believe that the US economy is structurally dependent on exploitation and consequently needs a steady stream of non-white immigrants to extract labor from. (Maybe for example he believes that the children of immigrants are less exploitable than their parents because for example going to college in the US insulates a person from most forms of oppression.)
