There's lots of reasons it isn't so simple for people.
And one reason why it is extremely simple: if you're not from the U.S. to begin with.
Our universities and business culture lure the best of the best from all over the world to the United States. Our draconian travel rules, lack of privacy and "shoot first, ask questions later" laws discourage them from even visiting let alone live here.
Look like a Muslim? Suspected jihadist. Detained.
Carry more than a few thousands dollars? Drug dealer. Money seized.
One of the 99.99% that don't fall into any suspect group? Still suspected. Violate your privacy and freedoms.
If I were anything but a natural born citizen, this crap would make me leave in a minute.