There's a simple hard truth that intelligence is largely heritable and intelligence has a significant correlation to an individual's fiscal net effect (not only that individual, but his/her children, and the children's children, and so on). There's a level of low-skill immigration (or at least guest workers a la Asian countries) that is good because you allow high-skill people to work in high-skill occupations. But once you cross that level you just drag down overall productivity. What's happening in America is you have an extremely productive class of (typically white and Asian) skilled workers, and then an extremely unproductive class of service and public-sector workers that are increasingly a drain on fiscal health. The left doesn't see this differentiation (or they do, but they refuse to acknowledge it because noticing differences among groups is what bad people do) and just simplify it to "immigration good" like some sort of late night watching, reddit-reading NPC.