Hey. The reason for this is that it requires a Google certified OS, unfortunately.
I would recommend looking at this community maintained list:
I've previously heard that Curve when obtained from Huawei'd AppGallery does contactless payments which work on GrapheneOS, but I'm not sure if that's still the case. Same for Barclays in the UK and other options in countries like France etc.
There's nothing inherently broken about contactless payments on GrapheneOS, it's just that apps that require a Google certified OS won't work, which isn't specific to contactless payments or banking apps.
Apps could whitelist GrapheneOS if they wanted, and GrapheneOS provided a guide for them to do that:
I hope this helps.