Because there's already social safety nets those people can use. And yes, absolutely, there are probably going to be groups of people who are barely working now that will stop, but given there's already means to not work, I can't see this being a huge group, and really, this is an optimist / pessimist face-off, which is sad, because that's how we probably see the possible outcomes too. The only way we can know if it'll work or not is for someone to try it - which, thankfully, YC is.
Not sure if you've ever used those social safety nets, but they are not easy to qualify for and/or sign up for, so only the stubborn and/or desperate actually take advantage of them. There are also disincentives, like misinformation, run-arounds, and social shaming. From what I've heard, these reasons are exactly why UBI is a better strategy than those social safety nets--and why we can expect more people to take advantage of them.
