I think part of the reason is: (a) Parents will immigrate with their kids knowing that they won't have legal residence status. I know some immigrants are escap

I think part of the reason is: (a) Parents will immigrate with their kids knowing that they won't have legal residence status. I know some immigrants are escaping dire poverty or persecution, but not all. So I think parents have some responsibility here. (b) Plus, the US is relatively lax in enforcing immigration laws. People do (a) and get work, send their kids to school, buy a house, etc. There's no way you could get away with that in (say) Japan.