And what, living in other countries is so inexpensive that people can pick up and move easily?
It's more like those people are looking for a better way of life and have the initiative to make it happen.
I know a woman who came here from India with her family.
She was stunned to find that it took both incomes to keep a roof over their heads. In India, you can have a decent home with only one adult working in the household.
In India, when you are down and out there are, apparently, options besides being homeless. In some areas you can stay in what we would have called a "flop house". She hadn't seen anything like that here, and I haven't ever seen anything like that outside of old movies.
She and her husband are trying to save money to go back to India and finding it quite difficult. It's not like they have a huge mortgage and a bunch of credit, either. They both have decent jobs.
Each country has its pros and its cons. There's no reason to believe that people don't leave the USA because they get comfortable and can't match the standard of living. For many, other issues are more important. There is also no reason to assume that a lack of motivation keeps people here, either.
For every tale of people looking for a better life, there is a tale illustrating how stunned people from other countries are when they find out what daily life is like, compared to the country they left.
This isn't a value judgment, but something important to remember. It's not an either/or proposition and its not adversarial.
It's just the way things are.