I think the bottom line has to do with reciprocal agreements. If all of the vehicles were made strictly in the US trade agreements with other countries would be such that they would be only sold in the US. Other countries would start their own industries.
By sharing the workloads units made in the USA are sold in other countries while some of the compents or assembly is done in other countries for sale in the US.
Gordon and Janet
Tour 42QD/inTech Stacker