Stupid question: why didn't they add that part to the actual production cars? It doesn't sound like a complex system or problem (compared to say, reducing emissions versus just cheating in emission testing) especially since they already seem like they know exactly what they had to fix.

Even then I'd understand if it was a one time thing (rush in production or design, no time to redesign) but this is systematic and across a wide range of models. I guess the other models in question had more deeply rooted design issues that can't be fixed with a less intricate tweak?