RedPanda So is there nothing at all where you can simply add your own card for NFC payments?
It's not really a technical problem, more of a cost/risk/market problem. Google, like Apple, spent big bucks setting up a network and getting merchants to accept it. They want to limit their risk of that system being hijacked, so they feel it best to trust their OS and the systems of their contractual OEM partners.
Others, such as Samsung and Huawei, have similar networks. Some banks may choose to build their own apps rather than signing up with one or more large networks, but then they assume the risks.
Just because it's not technically hard doesn't mean it isn't real-world hard.