You need to see UBI as the rent humans get because they own the land on which productivity happens. In reality, when you "buy" land, you never really own it, because that would imply entirely absurd consequences. First, if you have true property rights over a piece of land, you have property rights over the people who live and work there, which is basically the feudal model of serfdom. When you "buy" land, you are actually buying use rights for that land. I am personally a big fan of the Georgist taxation model, where land use is the one thing that is moral to tax, because land is actually everyone's, also, it is a finite resource that an individual can't produce, so taxing land use is not exerting property rights over the labor of a human, the definition of slavery.