Most "third world" or "developing" countries aren't that bad, and there are places in the US far worse than the median developing country.
Also most people in most places do not want to go to the US, even to visit much less immigrate. It's generally either the worst of a particular society or those specifically harmed by the US previously and feel their chances are better off with the abuser instead of in the abused country. It's not a wanted destination.
This was a MASSIVE eye opening shock to me. You watch NCIS or any pro military show and they'll pan to Baghdad or anything middle east and you'll see crumbling buildings or warzone with a sepia filter. I was got smacked when I saw a real skyline photo of Baghdad, and istanbul, and most cities. Our media is dead set on continuing the thought of these empty deserts
There is a book called Factfulness where they talk about presenting the UN scientists their own data and surprising them at the standard of living in many third world countries. People's ideas of third world countries is based on what they were like in the 50s, but many are catching up to the developed world in leaps and bounds.
The UN General Assembly Human Rights Council 2018 report on USA's poverty and human rights is a pretty quick and clear overview which makes it clear that parts of the USA are just undeveloped:
I'm not sure where you're from, but at least in the Middle East that's not the case. It's a very desirable immigration destination here (less than Western Europe, though).
See the last part of the second paragraph for that. Victims of the empire paradoxically tend to want to immigrate to the empire believing they'll be better off there than in the country that empire targeted.