Because we already have a preexisting standard we conform to. Saying Jeremy Lin is a Chinese export is like saying the NBA mostly consists of African exports - we generally consider the black players with some African heritage as just Americans. Jeremy Lin is an American with Chinese heritage - This also helps to differentiate from players who actually have emigrated from other countries like Hakeem Olajuwon.
The term "African American" is complicated. But I think when someone self-identifies as a [country]-American, it's generally fair to call them an export of that country. You would want to be clear in certain circumstances that they weren't born there, but it's not a terrible way to phrase things.
Note that this is different from just having heritage in a country. There's immigration only a couple generations back in my family, but it's already mixed to the point that I wouldn't call myself any specific country-American.
Because most people don't view/phrase it like that and it sounds absurd either way. At best your phrasing is absurd and at worst (which you deny is the case), racist.
