A protocol to support decentralized payments is a hobby project for an individual. You give each institution an identifier (e.g. their domain name) and each institution gives each customer an account number. If you're account 123 at Chase then you sign in as #123@chase.com, tell Chase you want to send $5 to #456@bankofamerica.com, they send the money and Bank of America tells their customer they have a new deposit. If you want to collect money from someone, you tell your bank to send their bank a payment request and they can either approve it manually (e.g. so you can deliver the goods for a one-time purchase) or configure some rules for which accounts get approved automatically up to some threshold amount (e.g. for recurring payments). Also no reason for banks in different countries not to all support the same protocol.

That doesn't require a central bank or any centralized third party payments intermediary. All it requires is for banks to know how to send money to other banks, which they obviously already do and the specific implementation of that isn't even relevant to the customer-facing payments protocol. So how does this not exist? It can't be that no one wants it, so it's got to be that someone (e.g. Visa/MasterCard) doesn't want it.