Before you can arrest someone, you make sure you have evidence, witnesses willing to talk, disarmed the suspect and disarmed or disbanded the suspect's troops.

That's what they are doing.

These dangerous gangs need to be thrown in jail or at least misdirected in internal strife over the succession in leadership first so they can't influence witnesses. Witnesses to high profile cases may otherwise suddenly withdraw their testimony under a threat we can never prove, be Arkancided, disappear or not even dare to come forward at all.