We do have, to some degree, a UBI. These are benefits the government gives out to the poor, elderly, or disabled.
That is social security, not UBI. UBI by definition is universal. And, social security is means tested, meaning people aren't incentivised to work more, if they lose payments for doing so.
Imagine being able to backpack around the world knowing you'd be able to afford to eat wherever you go.
The UBI would be unlikely to get someone very far, and I would assume there would be rules like "live in the country".
For example, you'd still need a plumber to find and repair leaks, which are often in inconvenient places for machines to access.
How many people have repetitive jobs, and how many plumbers are needed? All jobs likely have some level of repetition that could be automated, so the impact is immense at scale.
We don't see oil production automated even though oil companies have the capital to do it.
It is a cost issue for now, but the cost to automate are coming down rapidly. If a company can do it cheaper with people, they will - but that won't always be the case.
And with a UBI, perhaps they would only take the jobs that interest them rather than have to take a job for the money.
This would be the hope.