On top of the mindset already mentioned there is also a fair bit of fear. A lot of perps these days are very well armed and in such a situation you have to hit hard and fast.
The problem is their intel sucks and so the police treat every situation as a deadly one.
If the police want to be militarized then they’re going to have to have a soldier’s ethos. As Maj General Sir John Hackett put it, “The essence of a soldier is not to kill, but to die.”