In addition to your third paragraph, auto makers are disincentivized toward adding security features because they CAN'T advertise them. If Toyota started advertising the all new Prius with the feature "your ability to brake won't be taken away from you while you're barrelling down the highway at 70 mph", consumers wouldn't just not care, they would question why the fuck that wasn't there in the first place. This sort of stuff is expected to have been there from the start by consumers, so at this point its nothing but a cost sink to add it.