I suggest looking into some basic experimental design texts if you're interested in why this is so important!
Not to mention that the amount given - a total of $3200 to childless people over an entire year - amounts to around 3 months of an average 1 bedroom apartment's rent in America in 2020, much less in cities, and not including groceries or utilities. As a young, childless person who got the full amount that year, it gave me exactly what I needed to be able to move out, but certainly wouldn't have sustained me - even in a "flop house" - long enough to really avoid looking for work.
We increased unemployment benefits to the point where for many people it was more than their previous income. Of course people weren't going to be excited to effectively lose money by taking a job. That's not at all a UBI which pays whether or not you're working.