Ok, let's say someone in the black market invents a portable NFC scanner to extract payments from unwitting pedestrians in crowded places. Apple pay takes a handful of security measures to prevent such hacks. Should Apple also open up its FaceID/touch ID interface to secure others' payments over its phone? Should it make the "secure enclave" open? Should it write software and hardware to allow third party payments to thrive on its own closed platform?
It's about how it has the potential to get exploited and how they don't want to be responsible for securing other companies third party payments. People specifically pay Apple money for a predictable, secure experience. App store has at least some quality controls in place that prevent malicious apps from getting released. In my mind, the NFC chip follows the same principle. Why should Apple open it up for anyone to do anything with it?
Sorry but you obviously don't have a clue what NFC is.