Why don’t the cops bring in the water cannons? It would disperse the group and sting them like Hell. Is there a reason why we never see these?
Yes, there is.
It will conger up images of what rat officials did in the past... usually accompanied by snapping dogs, at which point the police will be seen as being of the same ilk as old southern democrat racists who did it even to peaceful protestors.
There was a gadget someone invented for the Army, IIRC, which uses sound waves to make people lose control of bodily functions, kind of takes the bravado out of tough guys fast.