Because the outcome is a lot worse for society than unpaid rent if they don't have a roof over their heads.
New Zealand really loves to chase the wrong end of the stick and focus on benefit-bashing. The reality is that there's a much higher magnitude of tax fraud than there is benefit fraud, so how about we focus on the big ticket items first?
Agreed, if we move on from the topic about not paying rent and onto a discussion around violence/harrassment or destruction of property, that's a policing issue. KO as "landlord" could impose some penalties such as rehousing to a less desirable location or imposing fines, but throwing antisocial or violent people out onto the streets are simply going to make our streets full of more anti-social and violent behaviour that the public-at-large will then encounter. Longer term, we need to adapt our policies across the board to set the next generation up for success so people have less need for social housing and then everybody is better off. But in the circumstances we have today, it helps nobody to kick these people to the curb.