The simple answer is taxes. Companies don't get UBI since they're inhuman. And rich people will almost certainly pay more in taxes more than they receive in UBI.

> I'm guessing that we don't spend as much on that admin overhead as we would under a UBI plan that includes everyone.

My intuition says the opposite. Speaking in startup terms, it's significantly easier to roll out a new feature to all users, or offer a discount to all customers, or enact a policy for all employees than it is to create two or more groups to determine who gets what.

Though our guesses disagree, I'd hope we could agree to experimentally test the hypothesis, and to enact policy based on what the data show.