>>528202512
Nikke cores are the product of reverse engineering and can be iterated and improved upon using human intellect and uninhibited creativity. Rapture cores are limited to whatever coded manufacturing specs are used during their construction and can only be reactively improved based on data inputs.
This also assumes there are no external constraints to their improvement algorithms, because if you think about it for 2 seconds you start wondering why the raptures don't just evolve to become an unstoppable self-replicating nanite swarm.