More than half of PhD students in STEM programs are international students, often from developing nations, for whom the calculus is slightly different. A graduate student's stipend is probably comparable to the wages they could have made back home.
There are lots of great positives about doing a PhD in USA- salary/stipend is definitely not one of them.
The academic departments increasingly play important roles in immigration. They probably benefit from closer integration with government’s immigration organizations.
If anything, the immigration equation is far more relevant to the US than it is to other countries. A top PhD used to be able to fetch you an eb1 Green Card which is a lot faster than the eb2. However, 'eb' GCs have to be filed by your employer, which means leaving Academia or having a decent enough post-graduation industry outlook. With eb-1 requirements only getting more stringent, this perk is also becoming less relevant.
A poor country PhD student/grad in the US/EU still has to live under minimum wage conditions. The fact that the stipend might be considerable more money in their home country, does not necessarily mean that in PPP terms - phd grads are better off than say, a 'developed country' individual.
If you moved to a developed country with higher living standard costs with higher salary than your home country. Even though, you earned considerably more than your home country; the money might be nothing more than minimum wage in your new place of residence.
Comparing developed country humans to developing country humans's expectations just adds more fuel to the assumption that people from poor countries are happy to work for less. It applies the same for developed country humans moving from place to place and even states here in the US.