The reason is that the cops and most LEO’s follow the path of least resistance.
It is a lot less dangerous to disarm granny than it is to disarm some punk thug who WILL fire on the cops and isn’t afraid to die.
The police does not start a war with the gangs because it cannot win. If the gangs kill just one LEO every day, from a safe distance or from a prepared ambush, the killers will not be caught, but the morale in police's ranks will drop so low that people will be quitting en masse - nobody wants to die for no reason, after all. So the police simply knows its limits.