Because the hijackers came in legally via other Western nations--they didn't hop across the Rio Grande.
There's a reason that they opted to not do that.
Not gonna mention that 15 of 19 were Saudis -- NOT courtesy of "Western" nations, however, they could just as well have brought in a small army through the southwest border, aka The Securty Sieve.
Another interesting point: In your opinion, shall Homeland Security have the right to "racially profile"?