Xenophobia and racism, mostly. And yes, it’s a solution to the aging demographic crisis many countries face (at least in the medium-term).
I remember seeing a video of a presentation back in the Bush years by some neo-con group that advocated for immigration to Pentagon or DoD officials or something. The argument for immigration was mostly the same: we have an aging population, so we could integrate immigrants (who are statistically younger) to solve this issue. I didn’t agree much with the broader idea of the presentation though. The broader idea was that there were still some parts of the world not a part of the global U.S.-led hegemony (mostly the middle-east and Africa), and we must spread democracy and capitalism to them. The argument was that globalism/capitalism ensures peace, and that both WWI and WWII happened because globalism was falling apart shortly before those wars. So, to ensure world peace, we need to globalize the entire earth and bring all countries into the the U.S.-led hegemony, even if that means starting wars to spread democracy, lol.