But not everyone would. I really love my work. Fewer people in the marketplace would be good for me because it would drive up my wages, which means I could afford to do things that the average UBI recipient could not.
Also, it's not like UBI would solve all social problems. Many people who like to work in nonprofits today would continue to do so. Probably even more people would because they would have more of a safety net underneath them while they save the world.
That said, I'm skeptical of a UBI. Too many unknown consequences.