Because then you need to test for means, and that means you have to hire new bureaucracy to conduct those tests, and then pay their benefits and pensions and travel expenses, and then you need to hire additional bureaucracy to address occasional loopholes in your checks, and then 10-20-50-100 years later you're back to square one with massively bloated self-serving bureaucracies conveniently appropriating dollars to save pennies.
That's what drives UBI advocates bonkers about the current benefit programs. For example, food stamps benefits. I say anyone with an SSN should be able to apply for one and get it electronically. It helps farmers, helps food availability, and is generally spent locally. Instead we have layers and layers of bureaucracy ensuring Jeff Bezos and Bill Gates are prevented from receiving SNAP benefits.