Because they are not mentally equipped to deal with the situation and they panic.No, you would have stepped backwards so that he couldn't continue to take your gun whilst he was on the floor flailing about.
You wouldn't have shot him, no one else would have been injured and they didn't have to pull their guns out and fire at him.
There really wasn't any need for 5 officers to be brawling with the guy, and the speed of which the shots were taken demonstrates the intent to kill, not to wound.
If they had the required training they would be able to handle these situations a lot better.
all cops should not have guns and a lot of people shouldn't be cops in the first place