People initially think of poverty, but that's really not a big deal for the vast majority of the world. Pakistan is about as rich as Moldova. India and Nigeria are richer. China is about as rich as South America, which is about on par with eastern Europe.
Virtually everyone makes way more money than they need to live by. Until you get to true hellholes like Haiti and Somalia, life is pretty good in the modern age.
War-torn countries are obviously hellholes, but they're pretty few and far between these days. Syria may be the only hot war on Earth right now.
What really makes countries worse to live in than the USA is the crime and corruption. People can't feel secure in their persons or property. Whether it's South Africa, the U.K. or Sweden, they're all hellholes because rapists are always just around the corner. Mexico is a hellhole because drug lords keep shooting up the place, not because of poverty -- $20k a year per capita is a perfectly middle class life style.
Brazil is plenty rich -- but it's not safe. Pollution is out of control, just like in China. Crime is out of control. All the government officials are corrupt. In Argentina, the GDP is fine, but you never know when all your bank assets will be seized or when the government will default on all its bonds and pensions and every other promise it makes.
The best definition of a hellhole, then, is a place you can't feel secure in your person or property. This covers a much larger portion of the world than just 'poverty' or 'war-torn' would explain.
Russia and Poland have about the same GDP per capita, but Russia is unsafe whereas Poland is extremely safe. In Russia, extra-judicial killings are common, regular crime is high, terrorism is high, and corruption is universal. Elon Musk tried to get rockets from Russia but gave up and started building his own in California because he realized it's impossible to do business in Russia. Your property and contracts are never secure. The life expectancy at birth of a Russian male is 65. In Poland it's 74.
The reason you don't let endless waves of third worlders into your country isn't to preserve the economy (the economy gets by even in 90% black South Africa), it's to preserve that wonderful blessing of safety that only a few countries on Earth possess. Places with rule of law, property rights, stable currencies, a democratic process that precludes endless civil war power struggles between rival parties or tribes, low crime and low corruption. Places with pollution controls, basic sanitation and health measures.
Third worlders are not in the habit of being safe. They are high crime, high corruption, high pollution, low sanitation, high terrorism, low respect for democracy, high disease (like AIDS in Africa). When they come they bring many problems with them. It isn't even their low IQ or low per capita GDP. The modern economy can easily absorb such minor issues. It's the death and destruction they directly visit upon their hosts, and far worse, their voting habits which indirectly cause the exact same terrible governments that ruined their old countries. For this reason illegal immigrants are a lot better than legal immigrants -- at least they can't vote, and if they do crime, they can just be deported. It's the legal immigrants who keep doing terror attacks, voting for a literal mass murdering Satanist, and trying to recreate the failed policies of socialism that ruined Venezuela.
If third worlders were poor but peaceful, like, I don't know, a bunch of Jains, and weren't given the right to vote, I wouldn't really care if a billion of them immigrated to America. But that isn't the option we're being given. Instead we're importing MS-13 and giving them the right to vote for Satan incarnate, who they then vociferously endorse, while attacking Trump rallies physically, women and children beaten and maced for just siding with a mainstream Republican winning candidate.
It isn't about hatred for the color of people's skin. It's wanting to live a nice, safe, peaceful and secure existence. It's wanting the opposite of the Chinese curse: "may you live in interesting times." These immigrants are nothing but trouble. They cause nothing but trouble. Everyone was safe and happy until the Somalians came in and started trashing the place. Rochester and Malmo were better before the gang rapes and grenades started going off.
I could care less if our wages go slightly up or down. America is filthy rich, making nearly $60k per capita. I just want to be able to walk the streets without looking behind my back, I want to be able to speak my peace without being physically attacked, I want to know that my bank holdings won't suddenly be frozen and expropriated like they do in South America, I want to know hyperinflation won't suddenly destroy the value of my dollar holdings like they did in Venezuela. I want everything around me to stay stable, secure, predictable and safe. I want any changes in my life to be caused by my own actions and decisions, with everything else playing out in a predictable pattern of behavior that doesn't undergo any massive swings from year to year.
I want to live a life of ordinary routines, like football in the fall, the Olympics every four years, new anime every season, and family gatherings at Christmas. Gunshots, gangsters, terrorists taking down skyscrapers or tribal warfare need not apply. This is what makes America great, and what makes the rest of the world a bunch of holes. The inability to go about your life in a worry-free, predictable, navigable manner because violence and chaos is always just around the corner.
The best way to improve America is the citizen's dividend, because it guarantees safety and security for all. All my policy proscriptions are geared towards the same end. Married life is more secure, and children raised by married parents are more secure. Everything is normal and predictable and safe as you go about your everyday life. Your brain stays on an even and healthy keel so long as you avoid drugs and drink, so people should avoid drugs and drink. How can we make life safe and easy so that people can concentrate on simply enjoying it? That should be the idea behind every single policy, not just immigration policy. But there's no better way to throw all of your safety away than open borders. You can kiss your country goodbye, and all the safety you'll ever have, for all future generations, the moment you surrender that.