It would be too hard. It would take too long. It would cost too much. We can't find enough cheap labor. We can't deport millions of people. We can't build a fence. We can't spare 2% of the National Guard to guard the nation. We can't allow them to stop anyone. We can't call them illegals. And for Pete's sake we can't call it amnesty, because all we're really doing is converting illegal aliens into citizens.
But don't let that get you down. There's plenty we can do. We can...
...send millions of people and our engines of war to the far reaches of the globe.
...rebuild New Orleans.
...explore space.
...make mind-numbingly expensive bad movies.
We can do whatever we want, whenever we want, for as long as we like. That's what arrogant imperial superpowers do.