Actually, they are; by shipping their uneducated, illiterate Mestizo underclass to us.
Notice the ones coming across the border aren't the lighter-skinned, Spaniard-looking ruling and middle-class Mexicans? The ones who speak Spanish and English? That's because they're happy in Mexico. They're doing just fine. And they'd be happier if there were less Mestizos to deal with.
This is very true. They are trying to get rid of the mestizo class. You never see any light skinned castilians coming across the border.