Maybe it's easier to understand this from the other side.
Suppose we have some kind of assistance programs for food and housing. If you don't make enough money you can get a card and with this card you can buy specific kinds of food, but not alcohol and not restaurant food. You get housing assistance but you have to live in designated government housing.
Then somebody suggests that we extend these benefits to everybody. But that's stupid -- why would you want to pay taxes in cash in order to get a food allowance card which requires you to buy what the government says, or live where the government says, instead of just keeping your money and buying whatever you want with it?
So all of those programs have phase outs, because people who "don't need them" don't want them. They'd rather have the money instead.
A UBI is money to begin with. Lowering your tax rate so that you don't have to pay to give yourself a UBI is a no-op. It cancels out. The tax you would have to pay to fund it and the phase out you would need to not receive it are mathematically equivalent.
The difference is that by putting it into the tax rate, you can see what you're doing better. If you have a 25% tax rate and then implement a 50% phase out rate, you're effectively imposing a 75% marginal rate on lower and middle income people and then a lower marginal rate on higher income people. Requiring it to be set as part of the tax rate makes it more blatantly obvious how silly that is, and that's all. And then, seeing that more clearly, people might choose more sensible marginal rates than that, so that you're never taking more from people who make less.