The real reason they shoot to kill is because they don’t want the victim to sue. A dead man never sued, their families have a lower chance of getting anything though that has started to change with so much footage.
No, you can't just shoot someone in the leg to disable them. You have very high chances of hitting main arteries anywhere (arms and legs).
Shooting a human being is not like it is in Hollywood movies. Once an officer pulls out his/her firearm and use it they know it means killing the person. That's why they go through other means of less lethal force before drawing.
Edit: Spelling