With a pipeline, if sections operated autonomously but cooperated with each other, and one goes berserk, its neighbors will shut down, but they won't be damaged. The repair work only has to repair the one section.
People driving cars are essentially doing what is best for themselves individually (within the bounds of the law), and that ends up translating to something that works for the whole. With a pipeline, that might not work: If pressure gets too high in one area, it might take highly coordinated control across thousands of miles to bleed off contents into buffer tanks & ease pressure a dozen segments away.
I'm not saying that couldn't be done, I'm sure the SCADA systems could be isolated from each other in this way, it just seems like it would require a lot more difficulty with explicit coordination between technicians, not a self-organizing system such with driving on a highway.