Because it *USED* to be the best part of the planet to start off new. Even if the land you came from was awash in good things, the class structure and societal restrictions were driving motivators. Sometimes oppression or bad times. But the key here is "chance to start over".
Unfortunately, the US is riding on a mythology that has been long shredding. The anti-immigrant mentality hasn't changed - Americans (even new ones) have "hated the new competition" for 400+ years. But the opportunities for the small, the peasants, and the serfs have evaporated over the last 30 years as the middle class has vanished. We're rapidly becoming another country with essentially medieval structure - the underclass and the robber barons, one where even the craftsmen/professionals/intellectuals are not valued by the wealthy for what they can create.
I'm making sure both my sons are not "stuck in the US" when it comes to opportunities. They both have their passport, they're both getting education that translates overseas, they're spending time overseas as the possibility arises. My wife has a fairly secure job as a pharmacist.. but she can't practice outside the US because of licensing requirements, so we're a bit stuck til she can retire.