Because it won't work. Tell a company to pay higher wages for the same positions by bringing them home, and that'll push them toward automation even faster.
These jobs may return for a short while. But that only brings false hope. And then we're right back to where we are today, hurting even more people in the process. It's extremely short-sighted.
Here's just one article on the matter https://www.technologyreview.com/s/602869/manufacturing-jobs...
Also note that Foxconn has already begun automating their plants. http://www.bbc.com/news/technology-36376966 So it's already a demonstratably viable option for these companies.
What we really need is to bring _automation_ home and have some serious discussions around solutions for these real struggling people instead of spouting the hand-wavy "bringing jobs back to the US" rhetoric.