Because anyone who can swim, run, or climb is already in America.
Juan
Gracias.
They're big enough to run around in.
A flat white.
A grand piano.
In Laotian.
Do they follow territorial boundaries? If the Kaos Kommandos start a brawl in El Paso and it rolls over into Juarez, do they say "screw it, let Justice League of Mexico handle it"? (Sorry if this isn't technically a joke; it was my shower thought this morning and I thought it was funny.)
Madam President.
Anyone who can run jump or swim is already across the border.