You aren't missing anything. But I think you are thinking of this as just a tax/welfare system, no different than any other in history. It isn't.
Think of a (maybe dystopian) future, where 100% of all production is entirely done by automated processes. Processes that can even repair and improve themselves. What then?
Well, someone will own those processes, and be making a lot of profit, with zero labor costs. Do they have an obligation to society to pay for the infrastructure, police, military, etc.. if there are literally no jobs to be had, so no one else has any money at all?
We aren't there yet, but the reason UBI is being discussed and experimented with, is because that is the direction we've been headed for a long time now. US production and efficiency has been climbing every decade. But the gains in profit from that increased production, has not translated into more jobs, or higher paying jobs.
The majority of American workers have not had a pay increase, adjusted for inflation and buying power, in over 30 years. But that doesn't mean someone else isn't getting more money. The owners of production have had record profits. And the wealth and income inequality in the US is approaching historic highs as well. Almost as high as it was right before the great depression.
Or, think of it this way:
In the future, an owner of a factory may have two choices:
1. Employee 1000 people making widgets, make 100,000,000 in profit, Pay 75,000,000 in labor. Net profit, 25 million - minus some regular taxes.
2. Full automation, 0 workers. Make 100,000,000 profit, pay 75,000,000 extra in UBI taxes. Net profit, 25 million - minus some regular taxes.