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.
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.