Because if you look at some of the statistics, you see that of people who are undocumented in the United States, the majority of people have overstayed on visas, they haven't come through the border.
Porque si se fija en algunas estadísticas, verá que la gente que está  sin papeles en los EE. UU., es la que tienen su visa vencida y no ha ingresado por la frontera.