But the thing is that there's other ways of dealing with discontent in the working class. Sure, UBI is one of them, but there are less pleasant solutions (just do slavery, or have a well paid class of kapos, or sow internal strife by pushing an "it's the Jews/Muslims/trans folks" narrative with the media that you own), and history shows that the politically dominant class is mighty partial to that kind of solution.

EDIT: Maybe I'm too cynical, but as far as I can see the last time serious improvements to the material conditions of the working class were made was when the soviets were still around, and the threat of a worker's revolution still seemed somewhat realistic. I think the politically dominant class has become quite adept at suppressing that kind of thing, and UBI does not seem to be part of their preferred playbook.