Many (probably most) UBI supporters are not libertarian. MLK supported it, as do contemporary left-leaners like Robert Reich and Paul Krugman.
Take SNAP as an example: it lifts millions out of hunger, but its steep cliffs create work disincentives, its work requirements (in many states) keep needy families unfed, and it’s restrictive and stigmatizing. It also costs 10% in administrative overhead. Turning it into a cash transfer — even something like EITC — would save billions, and also address its other issues. A libertarian might want that savings to go back to taxpayers, but progressives would have a case for increasing the benefit size for recipients. I favor the split-the-difference approach to make it politically viable, and here still many current SNAP recipients would see their benefit size increase by hundreds of dollars per year.
Yes there’s a lot to do, but JG shifts the focus from what needs to get done to how many people should do it. If a robot comes along that can sweep the streets, a JG-driven government would be inclined to still force people to do it to get their check, instead of enabling each of us to do what matters most. Its only advantage over UBI is hypothetical: the idea that people value jobs over just getting a check has never been shown in studies, while cash transfers not dependent on work have had significant beneficial impact on recipients.