Extension cords are mostly manufactured by cable companies in fully automated facilities. Therefore the manufacturing can happen anywhere in the world. Usually companies make extension cables wherever it is most convinent logistically. So, there are a few companies that manufacture outside of China.
Southwire is also an American compnay. They make their extension cables in the US, Mexico as well as China. However they have a line of red, white, and blue cables that are always made in the USA. We suggest you look at the label or just stick with the red, white and blue cables in order to avoid ones made in China.