Because systems based on pure ideology don't work well in reality. Its the whole idea behind implementing a UBI in our quasi-free-market capitalist system.

The larger point is that not only is the idea of a UBI decent and moral, because nobody should worry about being homeless or starving in a rich society, but because its much cheaper the the alternative! Only someone completely blinded by ideology (or somehow profiting off of the existing system) would argue that its better to spend $50,000 dollars a year per homeless person to manage shelters and run a bureaucracy if simply giving that homeless person $25,000 a year and achieves a better overall result.