make sure that nobody is starving, at least in a literal sense

The point is, that it's already done without an UBI, by a much lesser scarcity than in previous generations, augmented by a very meager but existing aid system, that nobody is starving in the literal sense, and this allows them to choose a less responsible lifestyle. And it is very hard for those who try to break out of this lifestyle, they have to literally flee their peers. I know of Gypsies who did successfully try to break out and become medics or engineers, and they (especially, but not exclusively, girls) were bullied by their own families: "how dare you think you are better than us!"