There’s more to it than just the cables, there’s all of the companies that integrate with their phones. There are a ton of accessories for iPhone, and they probably have contractual obligations to maintain compatibility for some amount of time.
and they all pay for the licensed lightning cable, they would love for nothing more than using USB C instead, apple doesn’t even have to stop producing lightning cables. They just have to stop using them on phones.
This is like the equivalent of not being able to manufacture a car because you slightly changed the design of how the transmission mounts to fix the problem where the transmission fucking grenades itself all because “well maintenance is hard”