Aren't more than a fifth of PhD holders in the U.S. foreign born? The story of immigrants returning home is a lot different from a story of native-born Americans abandoning their country.
The problem is that since WWII, the vast majority of these foreign born scientists in the US tended to stay and become naturalized U.S. citizens. It was uncommon, if not rare, for them to leave. In fact, there were waitlists for scientists trying to immigrate _into_ the U.S.
The reason for this was very much economic. The U.S., the leader in scientific spending over many decades after WWII, could pay scientists more than how much their home countries could pay. I should also note that in the past, the U.S. was also much more accomodating toward foreign born scientists in regard to naturalization, as well as popular attitude.
Now on every other bulletin board in my building, there're flyers in Chinese advertising very well-paid positions in China, and I get to read about the growing percentage of climate change deniers and creationists in the U.S almost every other week in the NY Times.