It took years and multiple assembly plants to build all the cars that have a recall.
The tooling for high volume manufacturing of the lock cylinders has most likely been discarded and had to be re-made for this recall.
How long will it take to replace 1M+ lock cylinders through ~3300 dealers?
Plus the day-day operations have to keep going.
FWIW, I'm waiting on a lock cylinder for a '02 grand am.