About half of all illegal immigrants in the USA today are not Mexicans. While a lot of people pass thru Mexico to get here, a significant number fly in on tourist visas and never return.
Good visa enforcement needs to be done as well. We use computers to track every person going onto a flight at every airport. I just flew cross-country this past week; believe me—I was logged to get in the airport and then the plane—surely we can monitor whether or not someone has left or overstayed their visa?
We could catch and deport them all in two months if we’d hire Visa/Mastercard to find them and send ICE agents to them using UPS and Fed Ex.
Well...VISA could...and don't call me Shirley.