Answer:
Three reason why GM build cars in China:
China has lower labor costs. This is the main reason, a Chinese worker earns a lot less than an American worker, and this results in a substantial cost reduction for GM.
China has important economies of scale. The United States also has important economies of scale, but China may have the edge in some economic sectors.
Finally, the third reason is that China has very good infrastructure like trains, roads, and ports, and this facilitates the logistics for GM.
If the president wants GM to move production back in the US, GM could demand better infrastructure (labor costs cannot be lowered, and the economies of scale are already there), and perhaps other complementary incentives like corporate tax reductions, or even subsidies.