Because nothing else has worked.
Most people attempt to change a person or situation through reason and discussion, usually one-on-one. When this fails, frustration may lead to anger. This can go on for years.
Appeals to reason and one-on-one discussions rarely produce change in someone engaged in self-destructive behaviors.
On the other hand, an intervention that includes several people meaningful to the person, that is executed in a controlled and logical way, that focuses on changing everyone's behavior at least for the moment, is highly effective.