Suppliers have APIs. Agents can call APIs. So why is completing a purchase still the hard part?
Every supplier becomes another engineering project. Four builds, four sets of edge cases, four ways for a purchase to fail.
Adding the fifth supplier is not your project. It is ours, and it changes nothing in your code.
A supplier API is built for a developer who already agreed the commercial terms, already has an account, and is writing code by hand. An agent has none of that.
Every card transaction could in principle be a private arrangement between a merchant and a bank. It isn't, because agreeing the same rules over and over costs more than shared infrastructure.
Agent commerce is at the same point, about fifteen years earlier than it feels.