A highly skilled immigrant, who is going to pay tens of thousands of USD in taxes to the American treasury is forced to wait an average of 5-6 years before getting permission to immigrate to the United States. At the same time, an illegal immigrant, who pays ZERO tax to the American authorities can stay in the United States, if he is able to produce half-a-dozen anchor babies. This is the beauty of the American immigrant system.
You're right. It's actually very hard to get a visa in Europe, especially in some poorer countries.
An educated worker looking for a better job might be denied a visa, while a beggar from Mexico can just jump the fence and impregnate a US citizen