The basic argument for UBI is that means-testing is a humiliating waste of resources and it would be more efficient to just hand everyone a flat payment. Basically it's better to cut a cheque to the small number of Gates-es and Bezos-es of the world than to force a large number of people to waste time proving that they're not, and hiring people to make that determination, etc.
In theory, it could work. However in a democracy where people care deeply about how their tax dollars are spent, it's a very difficult pill. Witness the constant threats to remove even existing means-tested welfare.
Finally, it's become a bit of a techbro pie in the sky fantasy because they have designs upon creating a large amount of unemployment and would like to try and keep the pitchfork mobs away from their bunkers.
(Proceeds to write 5000 word insensitive essay anyway)