Because to implement this you need to negotiate with individual credit card issuers. Basically how this works is that your phone is being issued a virtual card with the keys locked inside the phone’s HSM. Then it can be used to make NFC payments just like any physical card. So you need 1. contracts with many card providers, 2. card issuance processes with these providers 3. huge amounts of compliance bureaucracy. At the end of the day it isn’t really worth it unless you are a huge company and expect to have tons of users or see it as an essential feature of your phone OS.