I imagine it might be a hardware limitation? I couldn't find any details about where it comes from. They might be storing private keys in physical hardware; but

I imagine it might be a hardware limitation? I couldn't find any details about where it comes from. They might be storing private keys in physical hardware; but then why does each card has to have its own private key, why not share all of it together? It's blatantly absurd to have that much extra security for something that's fundamentally insecure anyways.