We are in an economic system based on ownership, with an ever-shrinking group of people owning an ever-growing percentage of things and making everyone else rent from them.
As labor is replaced by AI and robots, more and more people will exist who are not needed in this economic model, and while there's no need for a few people to own everything... they're not going to give it up.
UBI is a stop-gap. It still leaves a small hereditary capital class in control, because they will be the ones in charge of who gets UBI and how much. Because it will not be 'universal' in distribution.
We can't dial back human labor, because we're heading for a time when so little human labor is required that an individual share wouldn't be practical.
We're still going to have scarcity - you're going to want a place to live, you're going to need energy to power things, you're going to need a share of resources to have those things produced for you. There is no 'post-scarcity' on these fronts.