China already has industrial hemp. They have domestic auto manufacturers competing for the domestic market, which is needless to say a *huge* market. People start up new factories in China all the time. None of your arguments explain why some wealthy group of guys in China wouldn't want to take this marvelous hemp idea and start making cars that would be cheaper and of higher quality than any of the local competitors.
You also ignore the point about being able to make individual components from hemp. Look at a market like China, where dozens of small auto manufacturers are looking for any edge that would make them more competitive. *If* it were possible to use the hemp that's already available in China to make, say, body panels or a unibody shell or whatever, that would be cheaper and lighter and stronger than the alternatives, why wouldn't some smart wealthy guy be out there taking that business away from the poor saps who are still trying to make those parts with metal?
It might be harder to make that kind of change in the USA, but if there's money to be made by doing it then do you think that "what about the steel workers?" is really the question that's going to come up in the board room? They've had to change the way they built cars before. They've started up new production lines to build cars in more modern ways without shutting down the old ones. Old production lines can be retooled, and if they can put more money in their pockets by doing so, what's stopping them? And if they really had the politician puppets in their pocket and getting hemp was the only obstacle to making more money, why wouldn't they just get those puppets to let them have hemp?
One explanation is that the free market in hemp-based automobile manufacture has been completely stymied in *every* country of the world, including countries where industrial hemp is available, by an automotive oligopoly that somehow extends its power into countries where it really has no significant control. And not just partially stymied, but completely stymied so that parts manufacturers around the globe don't even dare make components out of hemp in order to reduce costs, sell a better product at a lower price, get a larger share of the local market, and put more cash in their pockets.
An alternate explanation is that you can't make a body panel out of hemp that is stronger and cheaper than the alternatives in other materials, and ditto for any other parts that would be made out of hemp *somewhere in the world* if there were an economic advantage to doing so.