What's the point in giving it and turning around and taxing it away?
Because many people do not need the support. Letting rich people keep that unneeded money would just increase the income disparity and make less money available to those that need it.
Or, and I know this is a crazy idea so hear me out...
Or how about we don't give it to the people who don't need it?
Because there's no mechanism that achieves that goal without incidentally denying it to some people who
do need it; And because trying to work out who does or doesn't need it is itself a hugely expensive exercise, that is largely an inefficient duplication of the work of the IRS.
Pay everyone a UBI; Have the IRS calculate their income tax liability (in a greatly simplified way, as lots of tax breaks for low income people cease to be relevant or necessary); Collect the taxes (which in most cases will exceed the UBI payment anyway).
A person who currently pays $30k/yr in personal income tax might, under the new system, get a UBI of $30k and be liable for $60k in taxes.
That seems inefficient; but it's FAR less inefficient than having a huge bureaucracy devoted to trying to work out exactly who meets the criteria in your means test.