* Having debt helps keep a currency useful as a global reserve and store of value, with great soft-power benefits (though, honestly, the reverse is more true--being the reserve currency makes running up a debt easier).
* Having debt helps keep us out of capital-W War, as it binds nations together financially--if we owe another country a sufficient amount, they have a vested interest in seeing it come back. (Incidentally, historically, this is a shaky assumption as well.)
* Having a policy allowing for debt helps make re-investment in infrastructure and civilization-scale efforts more easy to accomplish.
* Having a debt helps encourage inflationary currency policy, which (in moderation!) has knock-on stimulatory effects on the economy. Some minor inflation is good for helping to encourage people to spend rather than hoard.
The more cynical take, of course, is: how can we not afford to pay UBI?
* Is it cheaper to remove, retrain, or simply resupply folks who are otherwise economic unproductive?
* Is it politically expedient to buy off the population whose only useful product is votes than to balance the budget?
* People are upset about work and making rent today...creditors of the US are some distant threat, and it's unlikely any politician will be in-office when that bill comes due--why hold off?
* We've spent more money on dumber things, what's wrong with this?
I don't exactly agree with any of the above points (in either category), but it's not hard to imagine people who do.