This policy isn't about trade. It's not even about automobiles, really. It's about hard power.

A nation's hard power in peacetime can be estimated (in part) by the number of tanks / planes / ships it has available, but in the event of a hot war with a great power, that static figure is matters less and less over time. In a protracted war, the far more important factor is the rate at which new tanks / planes / ships that can be manufactured going forward. Thus, in the context of America's strategic competition with China, our heavy industry is just as important a component of our hard power as are our existing stockpile of armaments.

If someone proposed that we send a sizable portion of our tanks/planes/ships over to China, we would easily identify that as an absurd strategic blunder. Why would you reduce your armaments stockpile while increasing that of your main strategic adversary?! Well, opening our automobile market to the Chinese would undoubtedly result in the equivalent outcome for armament manufacturing capacity. It would mean less domestic armament manufacturing capacity and more Chinese armament manufacturing capacity (subsidized by Americans!). To allow such a thing would also be an absurd strategic blunder!

We can all wish that the world was a peaceful place, and that the era of great power wars is behind us for good, but to mistake our wishes for reality would be very dangerous. Strength is the only source of security. This policy is a means of preserving what strength we have left.