When you know full well that you can shoot a man and suffer literally no consequences, why would you risk your well-being to try and disarm them without injuring them? Especially when you've been trained to see them as the enemy.
What's happening is the only possible outcome of the training and mindset that US cops are given. Add to that the fact they seem to hire for psycopathic tendencies, and you've got a sure fire recipe for civilian deaths.