If you were to try this, they would murder every last one of you – and harass or murder your families – even if you killed 5 cops for every one of you who died. (Just imagine the ending of Les Mis.) Hard for people to believe it’s worth it when the reply by the system is so harsh.
On top of that, there are just so many officers in a given city. Killing one or two or three won’t make much of a difference but it will certainly make the rest worse overall.