I'm not sure that's true, though most policies are implemented that way. If you get, say, a basic income payout that's a function of the income you make, that function scales all the way from 0 income/max UBI to >= max UBI income/0 UBI.
Not that that's like ... a policy slam-dunk or anything. Even putting aside the massive hurdle of getting UBI enacted into law, you'd still have to cope with sources of unreported income, asset/gift valuation, and so on. But that second category are, at least, tractable problems as evidenced by most countries' tax codes having contingencies to deal with them.
The U in UBI is that you give the same amount to everyone to avoid all that bullshit work of deciding who should get it. Then you raise income tax percentages so that people with a certain income are paying zero net and people with more income are paying into the system.
