Earlier quoted context omitted.
For starters, H1Bs are cheaper to hire. Also, they're more desperate to stay in the US, so they won't demand too much and won't walk away from poor conditions. That also fixes with the population issue since middle class Americans can't really afford to have kids.
Middle class Americans can absolutely afford to have kids. People in certain metros maybe can't afford it, but living there is often a choice they explicitly make, or at the very least talk themselves into. Sure you make half as much working somewhere else, but the cost of living is also a quarter as much.
I have no idea how other ppl here afford kids and manage to save. We spend $30k on day care and another $12k on health premiums alone.