Because a gun is an excellent example of force, coercion or violence. If someone points a gun at you, you know exactly what it means.

The laws, the courts, everything else is ultimately backed up by violence or the threat of violence. If you are convicted of a crime and you say you don't want to go to jail there are people whose job it is to MAKE you go to jail. If you cooperate, the threat of violence doesn't turn into actual violence but if you do not, it will materialize.

It's probably the easiest way to cut out the myriad different ways that the state will back up it's laws with actions and cut to the chase: you can choose not to comply but you will pay dearly for it.