Because there are plenty of people in the US who are neither immigrants nor the children of immigrants. In fact, they're probably a significant majority. So to have 8 out of 8 be members of the smaller set is rather unlikely.
Not when you consider that those people were pulled from a worldwide talent pool for a relatively niche topic. If you can recruit anyone you want in the world and end up with 8/8 Americans, that would be weird.
