Actually, it is not that out of reach that a person who is a professional such as an engineer or a doctor or lawyer to have $250,000 to be able to give to an adult child.
And it is possible for a poor person to encourage their children to study and become a doctor or engineer (see for example the Asian immigrant communities).
Thus a person who comes to the US poor, could have a billionaire grandchild.
And no other country offers this kind of opportunity at scale the way America does which is why so many people from all over the world endeavor to come here.