Funding a ubi wouldn't be that hard if we only cared about the kinds of things that get cheaper with time. Right now our federal tax rate is enough to cover 10k/year/pp (though nothing else), but if actually taxed the wealthy as much as we tax the middle class (for example, if we had a higher capital gains tax on public share sales, reducing the amount earned by the class of people who produce no real value and just earn money from stock growth), we could bring in enough to give out 5k/year/pp pretty easily. If everything that matters in life were getting cheaper the way computers and food did over the last half century, 5k/year/pp would be a good start, and as we got richer we could afford more.

But some sectors of the economy get more expensive with time, and those are what are killing us now and what prevents the UBI from being a good idea (before we fix them). Housing, Healthcare, education and childcare absorb more money every year and faster than the economy grows. Give people more money and those sectors just find ways to charge more. If we can't fix the way they do that, we get poorer in reality no matter how much more money we earn.

I realize this isn't how a lot of folks view things. To see it through my lens, imagine for a moment how wealthy we'd be if housing cost less than it did fifty years ago. If healthcare did. If education did. That's the world Keynes imagined, where we could be working 15 hour weeks (probably actually 25 hour weeks, but still) and still feel rich.