http://www.google.com/url?sa=t&source=web&cd=4&ved=0CB4QFjAD&url=http%3A%2F%2Freason.org%2Ffiles%2Fa87d1550853898a9b306ef458f116079.pdf&ei=ZUZJTImTIYP48AbjzZSPDw&usg=AFQjCNGEI6PLGi0XXzF8xa1WvSbBcyjMwA
As the image in this PDF will show you, legal immigration is far more difficult than it may seem.
Every legal immigration process takes between six and twenty-eight years to become a citizen, and unless you have legal family inside the United States or are very wealthy, you typically don't have a window at all. Otherwise, you're basically taking the flip of a coin, because green cards are capped well below the available demand.
So before you say "ILLEGAL IMMIGRANTS SHOULD JUST GET IN LINE!", you should think about why we have so much illegal immigration in the first place; it's because the legal process is tedious and often impossible for most people.