By having the lower priced Mexican labor build the vehicles these can keep getting cheaper without major investment in technology that is unlikely to pay for itself and without reduction in American worker's wages.
This also allows American Factories to make higher profits by focusing on making higher margin models without Ford having to give up a market segment.
Also, Security is generally better at American plants so by allowing the foreign factories to only build the crap models they are unlikely to compete on the unauthorized manufacturing market.
Also, This does put some money and infrastructure into Mexico so we can benefit as their country has slightly fewer people try to cross the border illegally.
This is simple division of labor. It lets Americans focus on more profitable things, there is no way this is bad for Americans.