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There's no technical reason why the US can't have a system like UPI. You're right there is no technical reason, but there is a very strong political reason.
They way things are now, people in the US have to pay for a lot of things that people don't pay for in other countries. Bank Transfers is just one example. Health Insurance (and care) is another. Higher education. Filing taxes. This list is endless.
Making people pay for this stuff is extremely good for the economy, and probably a big part of the reason the USA has the highest GDP.
It also makes many, many hundreds of billions of dollars for those entrenched in those industries, who will spend tens upon tens of billions of dollars lobbying to make sure it never changes.