Because the libertarian worldview is not complete. They get lots of things right (incentives, human action), but authority is part of the way the world works. You can either fill that vacuum with legitimate, well-defined authority, or with illegitimate, ill-defined authority. There is no such thing as no authority.

Thanks for asking btw, it's an important question.

Compared to a libertarian I'm a rabid statist. Compared to a statist I'm a rabid libertarian. It's a matter of perspective and principles.