Who, exactly, doesn't understand that the US university system requires a certain number of grad students to have the research output it produces, and a great way to close that gap would be to improve secondary education in the US? Also, I have a bit of ill will toward US universities that turns down US students in favor of Chinese students. I'm looking at you, UCSD.
There aren't many students fit for grad school (in the sciences) that are held back by poor secondary education. Obviously, some specialized programs could accelerate top kids farther, faster, but they're mostly already grad-school capable.
Not all high-performing students will do well regardless of their environment. Some will crash and burn without support, and that doesn't only mean "accelerating them farther faster". It can be enough to group them together with other kids that are similarly enthusiastic and curious, so that they don't shoot each other down.