Because pointing a gun at someone is a very dangerous thing to do. It massively escalates the situation. At that point, if they or anyone else in the situation

Because pointing a gun at someone is a very dangerous thing to do. It massively escalates the situation. At that point, if they or anyone else in the situation thinks they can shoot first, it becomes a real option and is also the only way to definitely protect yourself from the gun-pointer. Even if it's not a real option, people under that kind of sudden pressure don't think straight. A police-officer who habitually points a gun at people in situations that don't justify it is a danger to themself and everyone around them; the unlucky person staring down the barrel, other police-officers and unlucky bystanders in the wrong place at the wrong time. Once a gun is pointed at a person, there are no more steps of escalation, no more ways to express how much you want them to do something; the only next step is to pull the trigger. The situation has suddenly become extremely dangerous and very volatile.