No one's figured out how to pay for a UBI.
That’s not
exactly true. It depends on the exact details of the UBI.
There was a version of it posited a few years ago that converted the all tax money earmarked for all kinds of social programs into UBI, eliminating all of the government agencies involved in the administration and oversight of those programs. Most loopholes involving the sprawling welfare system would be closed. The only government expansion would be a subsection of the Treasury tasked with the accounting and disbursements. In terms of dollar value of benefits, this program delivered more benefits to the average recipients than the benefits they got under current law.
It was picked apart by both sides of the political spectrum.
On one side, reducing everything to a simple $$$$/month looked like paying people to do nothing- a big violation of the Protestant Work Ethic. To that mindset, UBI looks like it encourages social parasitism. (RW research on this has revealed mixed results.)
On the other, the UBI itself is anti-paternalistic. UBI depends on recipients taking personal responsibility for how the money is spent, and some current programs were designed to help people who have histories of questionable judgement. If a program’s key goal is to mitigate the negative societal effects of bad decision making, UBI looks like a recipe for disaster.
For both sides, imagine the fallout from telling your constituents that the entirety of the “welfare system” was being ended. The resulting clamor would drown out every sentence after that, and the helpful reports you printed detailing the new and improved system would become torches & fuses for Molotov cocktails.
So, even though that version of UBI was technically more efficient, there’s reasons in both sides of American politics that would undermine the political willpower to enact one in the foreseeable future.