Because UBI studies found it makes people perform significantly worse than without it. They didn't get better. Humans require toil, we require pain to function.

Matrix was a good point to this. They made the world a utopia and people couldn't handle it. Our monkey brains require pain give us a utopia and we'll just walle ourselves to death.

What UBI studies found this?

(The ones I've heard about, I'm fairly sure, didn't find anything of the kind. Which isn't to say that they show we should have UBI; there are big gaps between what has been tested so far and what an actual economy with UBI would look like.)

I've never heard of a UBI experiment that limited "toil". Nearly all UBI studies I've come across show an improvement in life satisfaction, reduced homelessness, drug use, etc.

I think you've taken the "work less" that was found in some studies to suggest they needed "toil". They simply found it somewhere else.