But your competitors can't really steal it, because they are subject to the same rules and have to make their code available too. So any theft of code will be apparent.

Manufactures will probably check each others code to make sure none of their own code has been stolen and file lawsuits if it has.

There's still China you know. They don't play by the rules and copy everything relentlessly.

Why? Competitors are the best placed entities, in terms of both resources and motivation, to perform a review.

Also, do you want officials to have to review and test one hundred different implementations of a safety critical piece of code, or would you prefer an evolution to a single standard shared code base, shared between manufacturers, for which all testing efforts are directed at new patches?

Before cars had software in them, competitors could learn everything by just buying a car, taking it apart, and measuring all the pieces. Yet that was the golden age of the auto industry, with no lack of competition and market growth. So I think it's not a problem for competitors to gain access.