Because we are humans and some of us won't consent so it must be forced. There are people who feel that rape, murder, and arson are all things they should be allowed to do. They don't consent to the laws. That's why they are in prison. It's a necessary thing, assuming you want a functional society.

They didn't consent to the laws, removal from society is already necessary for them. Forcing their consent doesn't make them any more likely to follow those laws.

You mean by moving somewhere else?

The point of laws in a democracy is that they're (theoretically, without corruption) consented to by a maximum amount of people. If you don't have rulers, then whatever your group of 10 people consent to as an unwritten rule will be broken by me, much to my gain.

By being removed from society somehow. Democracy still works, you can consent to accept certain laws the majority impose. And I don't see how a ruler changes that last point, if you don't consent to the laws those ten don't have to put up with your crap.