A lot of undocumented workers are already in the US  According to data from Department of homeland security most of the people who are here illegally arrive here legally and then overstay their visa. Over 400,000 people whose Visas expired in 2015 were still living in the country in 2016. That's about the same as the number of people who cross the southern border every year (328,00 - 479,00). The wall would do nothing to the people that are already here.