Android essentially provided bunch of interfaces over time to either load a complete payment infrastructure through hw means (usually it involved telco cooperating with banks to provide EMV secure element on the SIM card, and the NFC chip has hardware connection to the SIM card), and later and more popular there's both complete Host Card Emulation capability (so you can run EMV protocol in your app) plus Google Pay integration making it easier on issuer because now they don't have to implement EMV themselves on HCE, and also provide more convenience to users vs forcing you to open specific app when you have more than one card connected.
Ah, so it's the integration with issuer banks in the background that requires a lot of effort. That makes sense!
Thanks so much for enlightening me! :)