Because working ensures that you're doing something that is useful to someone else.

"Most people want to do something of value."

A universal basic income is kind of this wierd bourgeois selfish projection wrapped in this veneer of being altruistic: To be 'freed to do whatever one prefers' is really saying "I want to do what I want, regardless of whether or not it's socially beneficial".

The wierd thing about free markets is that although one can be selfish and money-grubbing, even the most mendacious and stingy person must do something in the service of another to accrue capital.