It's not like Mexicans buy much of anything that's made or grown in the U.S.
America's exports to Mexico in 2015 amounted to $236.4 billion or 15.7% of its overall exports.
1. Machinery: $42.1 billion
2. Electronic equipment: $41.1 billion
3. Vehicles: $22.4 billion
4. Oil: $18.6 billion
5. Plastics: $16.5 billion
6. Medical, technical equipment: $6.7 billion
7. Iron or steel products: $5.2 billion
8. Organic chemicals: $4.8 billion
9. Aircraft, spacecraft: $4.3 billion
10. Iron and steel: $4.1 billion
Not sure if it still holds true but in recent years Mexico was also the largest export market for American wheat, corn, pork & poultry.
You can argue that NAFTA hurts America overall (I don't feel qualified to argue either way) but there are definitely Americans who benefit from it.