Some back of the envelope maths suggests that to pay every American adult 1k/month would require 3tn USD per year, or about 11% of total GDP (12K per person per year, 258m adults -> 3tn dollars, total GDP around 27tn). That's about 3x as much as what the US spends on its military. And 1k / month doesn't strike me as a big amount.

I just don't really see how people think UBI is realistic in the current budgetary structure.