How are you accounting for the increased economic velocity of a sustained cash injection across the entire country?
Providing a "cash injection" doesn't require the UBI. It only requires taxing the rich. But the rich - yes, they mostly CAN actually leave the country to escape taxation, and operate from elsewhere, they can move their assets - increasingly virtual such as intellectual properly - onto offshore entities too... So there's really no option of taxing the rich.
I'll rephrase in case I've been unclear.
How, or have, you considered that if everyone gets an extra $1k/mo that a huge swath of Americans will just spend it every month? When they spend that money it does not disappear. Some goes to taxes and gets redistributed there. Some gets taken by the service providers (grocery stores, gas stations, cable/cell companies, etc) some gets vacuumed up into the murky world of private equity, some flows into the black market, etc, etc, etc. That money keeps flowing and keeps generating tax revenue. This is economic velocity.
The question I have is how have you examined this effect and found it to be unlikely to spur an increase in quality of life for tens of millions of Americans?
... except that part where $1,000 per month per person in the US comes out to about $4 trillion, which is 100% of all taxes collected.
There is no universe in which this much money can be given to every citizen in the US without creating new money.
Doing the accounting in a naive way is also misleading with a UBI, because it's effectively a tax credit. If you paid $15,000 in taxes and received a $12,000 UBI, you're really in the same situation as if you paid $3000 in taxes and had no UBI. The fact that you're nominally paying $12,000 more in taxes doesn't actually affect you because you get all that back.