After WWII, the other manufacturing centers of the world were rebuilding from the war, leaving the U.S. as a manufacturing superpower. Post-war Americans had pent up demand and bought lots of goods. This allowed U.S. manufacturing to flourish. Later, those countries rebuilt and third world countries developed manufacturing. Allowing low-value manufacturing to be done in places like China allowed the U.S. to invest the money made into high-value things. Now, manufacturing is highly automated, so if low-value manufacturing returned, it would make everything more expensive and not bring many jobs because manufacturing doesn’t require many laborers.