The end reason is always market dynamics. Certain people can have certain rationalizations but the market doesn't lie. Note that it's not a perfect market because of things like immigration control and artificial friction via the broken IMO interview process.
There are many people in China/India who are smarter and harder working and more driven than many people at FANG headquarters. However they're competing with each other in a highly competitive area while in the US there is less competition because of moats dug to create better quality of life. Like how elevators beep when there are too many people to prevent crowding.
So the end reason is very very similar to why any sort of inequality exists at all, just in a more extreme and jarring form because the field in question is supposed to be THE meritocratic field. (Which it is in a way... But most of that disappears in large companies. And in the truly free economic playing ground it's not about smarts but about money and smarts)
There are many people in China/India who are smarter and harder working and more driven than many people at FANG headquarters. However they're competing with each other in a highly competitive area while in the US there is less competition because of moats dug to create better quality of life. Like how elevators beep when there are too many people to prevent crowding.
So the end reason is very very similar to why any sort of inequality exists at all, just in a more extreme and jarring form because the field in question is supposed to be THE meritocratic field. (Which it is in a way... But most of that disappears in large companies. And in the truly free economic playing ground it's not about smarts but about money and smarts)