At NPR today is an analogy on why people find themselves forced to come to the US without papers:

Imagine immigration, especially from Latin America, as a two-lane residential street with a 20-mile-an-hour speed limit. Over the decades, it’s grown to an eight-lane superhighway. But the speed limit is still 20 mph. That is, visas for needed workers haven’t risen along with the traffic.

That is, even though people want to move into the country and employers here need immigrant workers, there is no existing line for the vast majority of people who want to come to the US legally.  Which is why next year, activists and advocates for immigration reform will be pushing for new legislation that creates a path to citizenship for the aspiring Americans already here—and creates a better system for future generations of immigrants.